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1 Published by the Oikos Editorial Office Nordic Journal of Botany Origins and nature of vessels in Monocotyledons. 14. Vessellessness in Orontioideae (Araceae): adaptation or relictualism? Sherwin Carlquist and Edward L. Schneider DOI: /j x, Volume 32, Issue 4, Pages If you wish to order reprints of this article, please follow the instructions here ALERTS Receive free alerts and stay up-to-date on what is published in Nordic Journal of Botany click here CONNECT WITH US Follow Nordic Journal of Botany on Twitter and Facebook Submit your next paper to NJBOT online at Subscribe to Nordic Journal of Botany and stay up-to-date with a wide variety of new findings of relevance to the botanical biodiversity on Earth. All Nordic Journal of Botany volumes since the journal started in 1981 are available as fulltext for subscribers online at Wiley Online Library click here To take out a personal subscription, please click here More information about Nordic Journal of Botany at

2 Nordic Journal of Botany 32: , 2014 doi: /j x, 2014 The Authors. Nordic Journal of Botany 2014 Nordic Society Oikos Subject Editor: Torbjörn Tyler. Accepted 18 October 2013 Origins and nature of vessels in Monocotyledons. 14. Vessellessness in Orontioideae (Araceae): adaptation or relictualism? Sherwin Carlquist and Edward L. Schneider S. Carlquist, Santa Barbara Botanical Garden, 1212 Mission Canyon Road, Santa Barbara, CA 93105, USA. E. L. Schneider edu), Dept of Horticultural Sciences, Univ. of Minnesota, Landscape Arboretum, 3675 Arboretum Drive, Chaska, MN 55318, USA. SEM studies of tracheary elements of subfamily Orontioideae ( Lysichiton, Orontium, Symplocarpus ) of Araceae show unexpected features. The plants are entirely vesselless. There are small pores in pit membranes of end walls of tracheids in roots and stems, but pit membranes remain intact. End wall pit membranes of stems have a coarse fibrillar texture, somewhat reminiscent of (but different from) those of Nymphaeaceae and Cabombaceae. Acoraceae, which are also vesselless, represent the first branch of the monocot tree, according to phylogenies, and the orontioids form the next branch. Vessellessness is therefore a potentially plesiomorphic feature in monocots, but it may also be related to the highly mesic habitats of Acoraceae and the orontioids. Various other non-submersed monocots have vesselless or near-vesselless xylem. Sectioned xylem of Orontioideae is also very suggestive of stages in the development of the pit membranes of both end walls and lateral walls of tracheids: open networks of cellulosic fibrils apparently precede the addition of denser fibrillar meshes, key information in assessing to what extent perforations in scalariform perforation plates of vascular plants may stop formation at the open network stage, and to what extent a thicker pit membrane experiences lysis and disintegration as the vessel element matures. Acoraceae, the family that is sister to the remainder of the monocots, have what must be called tracheids in roots as well as stems (Carlquist and Schneider 1997, Carlquist 2012). This, like similar conditions in Nymphaeales (Carlquist and Schneider 2009a, 2009b, Carlquist et al. 2009, Schneider and Carlquist 2009, Schneider et al. 2009), can be considered retention of a primitively vesselless condition that seems to have been shared by primitive angiosperms, judging from recent phylogenies (Soltis et al. 2011). However, secondary vessellessness in roots of monocotyledons is conceivable in the case of submersed aquatics such as submersed Alismatales (Carlquist 2012). This might be a response to lack of selection for vessels in a perpetually moist environment. The interpretation of vessellessness in early branchings of the monocot tree is not an easy one. Acoraceae have a basal position in monocots according to molecular data, yet they also have an aquatic or moist stream bank/lake margin habitat that minimizes selection for vessels, according to what we know of the correlation between ecology and vessel presence in monocots (Carlquist 1975, 2012). Absence of vessels in monocot roots could be interpreted as adaptive in aquatic or highly mesic environments in which rapid flux in conduction does not occur. The safety characteristic of tracheids (embolisms tend not to spread from one tracheid to another) is thereby retained. This condition could be retained from basal, now-extinct monocots or it could be acquired. This problem was considered for Acoraceae earlier (Carlquist 2012). Each group of monocots that is studied carefully with respect to tracheary element ultrastructure offers new information valuable for resolving this interpretational problem. According to molecular phylogenies, Araceae are, after Acoraceae, the next most basal group of monocots. Within Araceae, we have studied tracheary element ultrastructure in two subfamilies, Colocasioideae (Carlquist and Schneider 1998) and Philodendroideae (Schneider and Carlquist 1998). Although vessel elements seem clearly defined in some species of these two subfamilies by virtue of definable perforation plates on end walls, in other species, pit membrane remnants are present to various extents, suggesting a condition transitional between tracheids and vessel elements. The most important subfamily of Araceae to study in this regard is the subfamily Orontoideae (commonly known as skunk cabbages), because all recent phylogenies show the Orontoideae to be in a clade (which also includes Gymnostachys, subfamily Gymnostachyoideae) that is sister to the remaining Araceae (French et al. 1995, Cusimano et al. 2011). To be sure, the question of ecological correlation with occurrence of tracheids and vessels comes into play here also, because the orontioids can be characterized as plants of bogs and boggy margins of wetlands. The ultrastructure 493

3 of tracheary elements in the Orontioideae has not been hitherto studied (Keating 2002). The tribe Orontioideae consists of three genera. Orontium contains the single species O. aquaticum L., which extends from Texas into the southeastern US and northwards to New York and Massachusetts. Lysichiton contains two species, L. camtschatcense (L.) Schott (Siberia, Japan), and L. americanus Hult é n and H. St John (Alaska, British Columbia to California, eastwards to Idaho and Montana). Symplocarpus also has two species S. renifolius Schott (Japan, eastern Asia; segregate species sometimes recognized, e.g. S. nipponicus Makino) and S. foetidus (L.) Nutt. (Minnesota eastward to Nova Scotia, and southward to North Carolina along the Atlantic coast of North America). Material and methods The studied collections are as follows: Lysichiton americanus (supplied by Neal Bonham and Dr Sarah Reichard, Univ. of Washington Botanical Garden, Seattle, from the wild matrix area of Washington Park Arboretum); Orontium aquaticum (supplied by Dr John Wiersema, USDA, ARNS, material grown at his residence); Symplocarpus foetidus (furnished by Richard Gjertson from the Green Heron Bog at the Univ. of Minnesota Landscape Arboretum). Plants were thoroughly washed and fixed and stored in 50% aqueous ethanol. Sections of roots and stems were cut using a single-edged razor blade. Longitudinal sections were a prime source of information. In roots, however, somewhat diagonal longitudinal sections proved helpful in addition, because the tracheary elements are so long that end walls can be identified (Fig. 1) better and at least some pits can be seen in near-face view at the same time in somewhat diagonal sections (Fig. 1C F). Sections were cleaned in three changes of distilled water, dried on a warming table under pressure between pairs of glass slides (to prevent curling of sections), and examined with a Hitachi S2600N scanning electron microscope (SEM) according to the usual methods. The number of sampled individuals was small, as is almost always the case in comparative work where access to plants is limited, but more importantly, where extended studies of the xylem in a species reveal little variation and expenditure of more SEM times therefore becomes unwarranted. Within a plant, similar sampling considerations apply: if tracheary elements appear relatively uniform, as they did in the species studied here, views representing the range of variation observed are figured. The employed sectioning technique permits one to see several wall layers in pit membranes in those places of a section where wall layers are shaved away rather than intact. Such places are not uncommon, and must be taken into account in interpreting the nature of pits in end walls. One can usually find intact membranes in views from the insides of tracheary elements (Fig. 1A B), so whether pit membranes are porous, meshwork-like, or smooth should be determined from such views rather than from views of pit membranes from the outsides of tracheary elements, which may exhibit the scraping away of layers. Are the pores we observed and illustrated for end walls of tracheary elements representative of what occurs in the living plant? We believe so, because our observations are in line with what has been found by other workers (Carlquist 2012). The nature of pores in end walls can be cross-checked against absence of such pores in lateral walls, providing a confirmation of the validity of a technique. Size of pores in pit membranes that contain highly hydrated (pectic) materials may even shift within a living plant, according to ion concentration (Holbrook et al. 2002), and this may even provide a valve-like mechanism that governs conductivity of imperforate tracheary elements. Apparent size of pores as seen with SEM is also dependent on sectioning technique. The action of sectioning exposes larger pores in the more recently deposited wall layers. These features have been taken into account in the interpretations of the present essay. Results Lysichiton americanus Root (Fig. 1). Lateral wall pits of metaxylem tracheary elements are scalariform (Fig. 1A, bottom) or reticulate (transitional between helical and scalariform (Fig. 1A, top 2/3 of micrograph). The pit membranes of lateral wall pits are smooth and non-porous (Fig. 1B). End walls of tracheary elements (Fig. 1C F) could be clearly identified in some diagonal sections (Fig. 1C, E). Enlarged portions of those are shown in Fig. 1D and 1F, respectively. The views in Fig. 1C D are from inside a tracheid, so no scraping effect of sectioning should be present. Torsion, however, appears to expose some meshwork like patterns, which are therefore likely to be artifacts. In Fig. E F, the end wall portion of a tracheid has been exposed, the facing half partially torn away. Some parts of pit membranes are removed (Fig. 1F), with some remnants present (upper right, lower left). Porous structure is not evident in the remnants. Stem (Fig. 2). End walls of metaxylem tracheary elements have pit membranes that are porose and textured by a meshwork that is variously textured (Fig. 2A, C D). Views from the inside of a tracheary element show this condition in an unaltered state (Fig. 1A). As seen from the outer surface, portion of tracheary element end walls that are sectioned away show a layer that is network-like (Fig. 2B). The portions of the pit membranes that have escaped sectioning (Fig. 1A, lower left) appear to have few pores. Portions of the end wall pit membranes, as seen from the outsides of the tracheary elements (Fig. 2C D) are highly textured by fibrils and contain pores. The presence of portions of the secondary wall (Fig. 2C top, bottom) confirm that sectioning has not affected the pit membranes, only the secondary wall, and that the appearances shown in Fig. 2C D) are not artifacts. Protoxylem tracheid lateral walls are shown in Fig. 2E F. Although fibrillar texture is not evident at lower magnifications (Fig. 2E), one can see such texture in the primary wall in Fig. 2F. Strands of primary wall material that bridge the primary wall with the helical bands of the secondary wall can be seen in Fig. 2E F. 494

4 Figure 1. SEM micrographs of longisections of root metaxylem of Lysichiton americanus. (A) (B) lateral walls of tracheids, (A) scalariform wall facing parenchyma, seen from outside (below) and reticulate-transitional wall seen from inside (above), (B) lateral wall pitting seen from inside of tracheid, to show non-textured pit membranes. (C) (F) somewhat diagonal longisections to show end wall of tracheids, (C) end wall (center), traversing the semicircular space of two adjacent tracheids, (D) high power of portion of end wall shown in (C), tearing has exposed a network-like layer, but the pit membrane is basically non-textured, (E) end wall that has broken away from the end wall of an adjacent tracheid, (F) high power of portion of end wall shown in (E), with pit membrane torn away except at upper right and lower left. Orontium aquaticum Root (Fig. 3A B). Lateral walls of Orontium metaxylem tracheary elements are scalariformly pitted (Fig. 3A), with transitions to a reticulate condition (Fig. 3B). Although SEM images at higher magnification (15K or above) show some texturing in these walls, lower magnification does not reveal such texturing. Stem (Fig. 3C F). Metaxylem tracheary elements of Orontium stems have scalariform to reticulate secondary wall patterns on lateral walls. As seen from the inner (lumen) side of the tracheary elements (Fig. 3C), no porousness or 495

5 Figure 2. SEM micrographs of metaxylem (A) (D) and protoxylem (E) (F) tracheids from stems of Lysichiton americanus. (A) View of pit membrane from inside, showing pores in pit membrane that have presumably not been subjected to the scraping effect of sectioning, (B) three pit membranes showing open network of a layer exposed by sectioning; non-sectioned portions at left, (C) (D) pit membranes from end walls of tracheids, viewed from outside surface, (B) fibrillar texture between two bars (secondary wall portions, top and bottom) that have been partly sectioned away; the pit membrane surface has been altered little or not at all by sectioning, (D) pit membrane, higher power, showing fibrillar texture and pores, (E) (F) inside surface of protoxylem tracheid, (E) lower magnification shows a relatively smooth texture, (F) higher power shows fibrillar texture and bridges of wall material between the secondary thickening band (top) and the primary wall. texturing is evident. However, as seen from the outside surfaces of the tracheary elements (Fig. 3D F), pit membranes show pores to the extent that the sectioning has scraped removed portions of the pit membrane thickness. Symplocarpus foetidus Root (Fig. 4). Longitudinal sections of roots that were somewhat diagonal with respect to tracheary element orientation (Fig. 4A) revealed end wall pit membranes well 496

6 Figure 3. SEM micrographs of lateral wall pitting of metaxylem tracheids of Orontium aquaticum. (A) (B) tracheids from root. (A) scalariform lateral wall pitting; starch grains visible in parenchyma cell (right) adjacent to the tracheid, (B) scalariform-reticulate pitting of tracheid inner surface; pit membranes non-textured, (C) (F) tracheids from stem, (C) pit membrane at higher power; no fibrillar texture is evident (small tear in upper pit membrane), (D) portions of pit membrane from two adjacent tracheids, seen from outside of tracheid; pit membranes at left intact, pit membranes at right show exposure of a layer that is network-like, (E) pit membrane that has experienced moderate sectioning; a porose appearance of the remaining wall thickness is revealed, (F) pit membrane that has experienced degrees of sectioning from considerable (left) to moderate or little (right); the network-like wall layer is revealed, left. (see Material and methods). The area of Fig. 4A at bottom center, an end wall portion, is shown enlarged in the micrograph of Fig. 4B. The pit membrane shown in Fig. 4B is not prominently textured, but where the membrane and associated secondary wall is sectioned away, small pores are exposed (Fig. 4B, center, bottom) in a wall layer. The end wall pit membranes of Fig. 4C F are all probable end wall portions, seen from the inside (lumen side) of the tracheary elements. These show no evidence of sectioning, but they do show prominent pores in mesh-like patterns. All end walls of Symplocarpus root tracheary elements observed possessed pit membranes that are more 497

7 Figure 4. SEM micrographs of diagonal-longitudinal section of metaxylem root tracheids from Symplocarpus foetidus. (A) diagonal end wall of metaxylem tracheid; arrow indicates portion at higher magnification in (B), (B) enlarged portion to show that at the cut edge of the pit membrane, a wall layer with pores is exposed, (C) (F) end wall portions as seen from inside of tracheid. In each of these, pores are present. The tears ((C) left, (D) upper right, (E) left) may be associated with sectioning (on the reverse side of the sectioning), but the porose appearance is believed to be characteristic of the pit membrane in its natural state. intact and much less porose than those of vessel elements, and thus the roots of Symplocarpus we observed have tracheids. Stem (Fig. 5). End walls of stem metaxylem tracheary elements (Fig. 5A E) uniformly show pit membranes with a prominent fibrillar meshwork. The view from the inside of a tracheary element (Fig. 5A) shows that in a tracheid that has not been affected by the sectioning process, small pores are evident in the membrane. In Fig. 5B D, the outer surface of a tracheary element is shown. In Fig. 5B, sectioning may have removed some wall material from the pit membrane at right, resulting in exposure of larger pores. This may account for uneven size of pores in Fig. 5C also. In Fig. 5D, the secondary wall bars on the facing side of the pit membrane have been removed by sectioning (Two bars on the distal side are evident, center 498

8 Figure 5. SEM micrographs of metaxylem tracheids from stem of Symplocarpus foetidus. (A) (E) tracheid end wall portions, (A) view from inner surface of tracheid; pit membrane fibrillar, porose; appearance not altered by sectioning, (B) (D) pit membranes as seen from outsides of tracheids, (B) apparently intact pit membranes, left and center; pit membrane at right may have larger pores because of a slight degree of scraping of the sectioning, (C) pit membrane showing typical coarse fibrillar appearance, (D) coarse fibrillar appearance of primary wall occurs not merely on the pit membrane, but on wall portions underneath the secondary wall, sectioned away here (bars on facing wall evident by lighter areas, left and center, (E) diagonal view of a pit membrane (secondary wall seen sectioned, left). The pit membrane appearance is natural, and related to the coarse fibrillar structure, (F) lateral wall pit, showing very lightly textured appearance (compared to that of the end walls). and left). In the portion shown in Fig. 5D, the meshwork must underlie the secondary wall, because it continues seamlessly across the area from which two bars of the secondary wall have been removed by sectioning. A diagonal view is presented in Fig. 5E: the secondary wall adjacent to a pit membrane portion has been cut (note border), and the pit membrane, in diagonal view, shows a texturing comparable with the meshwork seen in Fig. 5A D. Portions of two lateral wall pits are present in Fig. 5F, seen from inside the metaxylem element. A fibrillar meshwork 499

9 less prominent than those on end walls of tracheary elements is evident. Discussion and conclusions Structural features The highly textured pit membranes of tracheid end walls as seen in Lysichiton and Symplocarpus are distinctive. The small porosities in them and the fact that pit membranes have experienced no lysis such as is characteristic of typical scalariform perforation plates are criteria for recognizing the tracheary elements of Orontoideae, like those of Acoraceae, as tracheids. Fibrillar meshwork-like texture is characteristic of pit membranes in end walls of tracheids in stems, but not roots, in the Orontioideae. These characteristics are somewhat reminiscent of the end wall pit membranes in tracheids of Nymphaeaceae and Cabombaceae (Carlquist and Schneider 2009a, 2009b, Carlquist et al. 2009, Schneider and Carlquist 2009, Schneider et al. 2009). However, the stem tracheid pit membranes in Nymphaeaceae and Cabombaceae are much more three-dimensional, and feature strands and layers superimposed over the pit membrane and some parts of the secondary walls. Interpretation of the textured pit membranes in either Orontoidieae or in Nymphaeaceae with respect to either function or taxonomic distribution seems premature to us at this point. More aroids and perhaps more aquatic monocots need to be studied in this respect. These types of textured pit membranes have not been reported in stem tracheid end walls of either Acoraceae or Hydatellaceae (Carlquist and Schneider 2009b, Carlquist 2012). Pit membranes of both lateral wall pits and end wall pits in Orontioideae begin formation with an open meshwork of fibrils, upon which a dense meshwork is superimposed. Sectioning reveals the open meshwork particularly well in the Orontium material studied here. Amorphous material as well as denser deposition of fibrils, is characteristic of the wall layering not just in Orontioideae, but in other monocots, such as Astelia (Carlquist and Schneider 2010). Studies with transmission electron microscopy of monocot tracheid structure have been very few, but should prove rewarding, based on our SEM findings. The pit membrane in a perforation plate in which the pit membrane will ultimately be subject to lysis and disintegration appears to us at this point as representing the first, open meshwork appearance, which one can see in the tracheids (arguably transitional to vessel elements) of Typha or Canna (Carlquist 2012). The degree to which a fibrillar network forms in the pits of a perforation plate as a vessel element matures is a vital topic for further investigation. The questions that need to be answered are: to what degree is a fibrillar pit membrane formed in a vessel element in which the end wall pit membranes are destined for disintegration as the cells mature and die? Do lateral wall pits develop an accretion of dense fibrils and amorphous materials while the end wall pit membranes are left as thin networks? Ontogenetic studies by means of transmission electron microscopy on the ultrastructure of vessel elements with scalariform perforation plates are much needed to solve this basic question. The answers may not be uniform, but may differ in particular groups of plants. Phylogenetic and physiological implications Pit membranes on the end walls of the tracheary elements in the studied Orontioideae were not markedly perforated; they are at most very small pores. The tracheary elements throughout the plants studied here must be called tracheids. As with Acorus (Carlquist 2012), the pores are about as prominent in end walls of stem tracheids as they are in root tracheids, but in both, the pit membrane is so little perforated that we cannot think of these as vessels in the textbook sense. The present pores offer only a small degree of increase in interconnection between the tracheids, and would increase flow only by a very small amount as compared to the open perforations of vessel elements. Secondary walls of these tracheary elements may show scalariform architecture like that of clearly discernible vessel elements, but the presence of intact (if porose) pit membranes in the end walls of tracheary elements promotes caution in terminological matters. Even lacking SEM data, Fahn (1954) used the term vessel tracheid. Hotta (1971) used vesselform tracheid for such tracheary elements in Araceae. Hotta (1971) was unable to find true vessels, using light microscopy, even in roots of Acorus gramineus as well as the aroids Arisaema album and Homalomena paucinervis, which he noted grow in wet places. Hotta was unable to find vessels in nodal roots of Rhaphidophora latevaginata and Pothos leptostachyus. Hotta (1971) also listed vesselform tracheids (wide tracheids with scalariform end walls, probably with pit membranes in them) for stems of more numerous aroids. One could entertain such a term as macrotracheid for such tracheids in these stems, as well as the stems of the orchid Vanilla (Carlquist and Schneider 2006) to stress this structural variation on the tracheid phenomenon. As SEM data has accumulated in monocots, pit membranes, variously porose, have been demonstrated in roots of such terrestrial families as Asteliaceae ( Astelia ), Orchidaceae ( Epidendrum, Phalaenopsis ), Philesiaceae ( Lapageria ), Taccaceae ( Tacca ) and Typhaceae ( Typha ) (Carlquist and Schneider 2006, 2010, Carlquist 2012). Cheadle (1953, 1963) doubts that vessels are present at all in the family Campynemataceae and in the apostasioid orchid Neuwiedia lindleyi. A number of other genera could be added to this list, judging from the illustrations of Cheadle and Kosakai (1971) for numerous Liliales (now mostly recognized under the order Asparagales). In the asparagoid genus Ophiopogon, porose pit membranes are present on end walls of what would be called vessel elements (Carlquist 2012) on the basis of light microscope studies, and the term neotracheid has been hesitantly advanced for such instances (Carlquist 2012). Such instances might indicate retention of the pit membrane in end walls as a secondary phenomenon, as suggested for the stems of Vanilla (Carlquist and Schneider 2006). A simple developmental change would be involved for this type of vessellessness: failure of lysis of the pit membranes in the end walls. 500

10 The above listings do not include submersed aquatic monocots, which mostly lack vessels entirely in roots and stems (Cheadle 1942, 1953, Wagner 1977). These, however, may lack vessels for a different reason: they lack late metaxylem, and late metaxylem is more likely to have vessels in monocots at large than protoxylem or early metaxylem (Cheadle 1953). Thus, phylogenetic loss of metaxylem could easily lead to secondary vessellessness in submersed aquatics. A number of non-aquatic monocots are suggestive of how this could occur: palms have vessels in metaxylem, but at least some have only tracheids in protoxylem and early metaxylem (Klotz 1977, data published in Carlquist 2012). The monocot families Petrosaviaceae and Triuridaceae lack vessels (Cheadle 1953) for a possibly similar reason: they are both mycoheterotrophic families. Th e tracheary elements with some vessel characteristics (scalariform end walls) but some tracheid features (presence of pit membranes in the scalariform end walls) are potentially well adapted to conduct moderate volumes of water, because of porose end walls. However, those same end walls are ideal for restriction of potential air embolisms to single tracheary elements. This scheme can be correlated with moist to boggy habitats that have only moderate degrees of seasonal drying, and with foliage that is not submersed, but does not have large transpirational capacity. This ecological description fits those Acoraceae, Araceae, Campynemataceae, etc, listed in the paragraph earlier dealing with terrestrial monocots that are vesselless throughout the plant body. That list is surely incomplete, and can be expanded more by means of SEM studies. The list assembled so far, however, does suggest a homoplastic distribution of the vesselless condition, because these families do not form a cohesive group within monocots (Carlquist 2012). Looking at the phylogenetic situation, as noted in the Introduction, Acoraceae are sister to the remainder of monocots; in that remainder of monocots, Araceae are sister to all remaining families. Within Araceae, the orontioids (including Gymnostachys ) are sister to the remaining Araceae. These placements have remained the same from the work of French et al. (1995) to the most recent global hypothesis for angiosperm phylogeny Soltis et al. (2011). Chase (2004) wisely warns I would caution readers not to be led astray by comments... that Acorus is basal within the monocots; this not particularly accurate description has been taken by some workers to mean that the genus has primitive traits for monocots, which of course is not necessarily true. No a priori reason exists for one of a pair of sister taxa to always represent ancestral traits for the larger group. The inflorescences (spadices for both Orontioideae and the Acoraceae are bisexual (rather than divided into portions of unisexual flowers, as in most Araceae) and have petaloid remnants. Both of these traits are commonly regarded as plesiomorphic with comparison to expressions in other Araceae. Certainly not all monocots that grow in the boggy, lacustrine, or very moist habitats where Acoraceae and where Orontioideae are found share the same vesselless nature. Aquatic species of Iris have vessels in roots, much the same as those in terrestrial species (Cheadle 1963). There has been a tendency in the past to regard vessel distribution organographically, systematically and with respect to scalariform versus simple perforations strictly as phylogenetic indicators (Cheadle 1942, 1943a, 1943b, 1944). As we

11 Cheadle, V. I Specialization of vessels within the xylem of each organ in the Monocotyledoneae. Am. J. Bot. 31: Cheadle, V. I Independent origin of vessels in the monocotyledons and dicotyledons. Phytomorphology 3: Cheadle, V. I Vessels in Iridaceae. Phytomorphology 13: Cheadle, V. I. and K osakai, H Vessels in Liliaceae. Phytomorphology 21: Cusimano, N. et al Relationships within the Araceae: comparison of morphological patterns with molecular phylogeny. Am. J. Bot. 98: Fahn, A Metaxylem elements in some families of the Monocotyledoneae. New Phytol. 53: French, J. C. et al Chloroplast DNA phylogeny of the Ariflorae. In: Rudall, P. J. et al. (eds), Monocotyledons: systematics and evolution. R. Bot. Gard. Kew, pp Holbrook, N. M. et al The dynamics of dead wood maintenance of water transport through plant stems. Integr. Comp. Biol. 42: Hotta, M Study of the family Araceae. General remarks. Jap. J. Bot. 20: Keating, E. C Acoraceae and Araceae. In: Gregory, M. and Cutler, D. F. (eds), Anatomy of the monocotyledons. Vol. IX. Clarendon Press, pp Klotz, L. H A systematic survey of the morphology of tracheary elements in palms. PhD thesis, Cornell Univ., Ithaca. Schneider, E. L. and Carlquist, S Origins and nature of vessels in monocotyledons. Araceae subfamily Philodendroideae. J. Torr. Bot. Soc. 125: Schneider, E. L. and Carlquist, S Xylem of early angiosperms: novel microstructure in stem tracheids of Barclaya (Nymphaeaceae). Aquat. Bot. 91: Schneider, E. L. et al Microstructure of tracheids in Nymphaea. Int. J. Plant Sci. 170: Soltis, D. E. et al Angiosperm phylogeny: 17 genes, 640 taxa. Am. J. Bot. 98: Wagner, P Vessel types of the monocotyledons: a survey. Bot. Not. 130: