2o8 On Secondary Thickening in Pteridophyta. ON SECONDARY THICKENING IN RECENT PTERIDOPHYTA. A OF late years evidence of the presence of secondary thickening in recent Pteridophyta has accumulated, and it does not appear altogether undesirable to gather the recorded facts together. In investigating secondary thickening in the plants of a family where the process does not usually obtain, some care has to be exercised in determining whether the elements which appear to be secondary in origin, really are so, and are not instances of a slow passage of elements derived from the apical meristem into permanent tissue. Of course, in those cases where a well-defined cambium is developed, this danger does not exist; a cambium, however, is not always present, hence the need for caution. To take a possible example ; inmature elements might be found in relatively old parts of a member, and may or may not be secondary. To determine whether they are of secondary origin or not, evidence of tangential divisions are to be sought for, and careful comparison made with the structure obtaining in the younger parts of the same member, where the tissues are still in a more or less merismatic condition. If no evidence of such a division can be found in any of the regions of the older parts under consideration, then the case would be what has been described above as a slow passage of elements, derived from the apical meristem, into permanent tissue. It is almost unnecessary to say that in all the instances of secondary thickening considered below, the requisite care has been exercised, and, in the mind of the present writer, there is no doubt that the records are those of true secondary thickening, although often of a very rudimentary nature. In the majority of cases, xylem is the chief secondary product, and the later-formed elements of this tissue may be distinguished by the following means: (i.) Position. The secondary elements of the wood are usually situated on the outside of the primary xylem, and in the older parts of the plants. The age of the member is of importance, and can be more or less accurately determined by the number of roots or of leaves, in the case of a stem for example ; and by the habits of the plant, more especially whether it be slow-growing or not.
On Secondary Thickening in Pteridophyta. 209 (ii.) Appearance, The secondary tracheides are often of somewhat slow development, and in many cases there may be seen immature elements with their walls partially lignified and still retaining living contents. (iii.) Relation to adjaeent tissues. There may be seen a more or less definite seriation, the elements appearing in radial rows. This character must, however, be used with caution, for the cells of the primary tissues may sometimes exhibit a radial arrangement. OPHIOGLOSSACE/E. Literature. Boodle. On Some Points in the Anatomy of the Ophioglossca;. Ann. Bot., Vol. XIII., 1899. Russow, Vergl. Untersuchungen. M6m.de l'acad. imp. des Sciences de St. Petersbourg. S6r 7, t. XIX., 1872. BOTRYCHIUM LUNARIA. Stem. The presence of cambium, and its activity in the rhizome of this plant, is so well-known that nothing more than the briefest description is necessary. It is sufficient to observe that the cambium occurs in a normal position between the primary wood and the phloem, and constantly adds new elements to these two tissues. The process is thus of a type characteristic of a normal Dicotyledon or Gymnosperm. Root. Boodle has shown that there is a considerable addition of secondary tracheides in the bases of the roots, more especially in those regions which are embedded in the cortex of the rhizome. The characters of this later formation of new tissue is of the same nature as that of Ophioglosstim described below. OPHIOGLOSSUM VULGATUM. Stem. An addition of secondary tracheides takes place on the outer margin of the primary wood of the older parts of the rhizome, and it is to be observed that there is no definite cambium. The later-formed elements may be recognized by their reactions with certain stains (see Boodle, loc cit.) Root. The phenomenon in question is similar to that which obtains in the rhizome, but the addition of tracheides takes place on all sides of the xylem, and not merely on the outer margin as in the stem. As before, there is no cambium ; there may, however, sometimes be made out a radial arrangement of the new elements; but, as has been pointed out above, this feature is not to be absolutely relied upon in determining which elements are primary and which are of secondary origin.
2IO On Secondary Thickening in Pteridophyta. The greatest addition of new tracheides is to be found in the bases of the roots, which fact corresponds with what has been found to obtain in the roots of Botrychiuiu. MARATTIACEyE. Literalnre. Fai-mcr, J. B., and Hill, T.G. On the Arrangement and Structure of the Vascular Strands in Angioptcris evccta and some other Marattiacea;. Ann. Bot., Vol. XVI., 1902. Hill, T. G. On secondary Thickening in Angiopteris evecta. Ann. Bot., Vol. XVI., 1902. ANOIOI'TEKIS EVECTA. Steiu. The secondary thickening which occurs in this plant, is of a somewhat more definite nature than what obtains in the Ophioglossaceie, for there is a fairly well-defined cambium formed by the division of the parenchymatous tissue surrounding the xylem. These merismatic cells do not extend around the whole of a vascular strand, but are local in their occurrence ; in the best examples, from six to eight such elements may be observed. The amount of xylem produced is small, being rarely of more than three cells in depth and this only in the best instances ; the more usual thing to flnd is the occurrence of single tracheides. Besides this intrastelar secondary tissue-formation, there is also a very regular cambial division in the endodermal layer. This activity leads to the formation of sometimes as many as six layers of cells arranged in a radial manner ; the products of the division remain parenchymatous, but the outermost elements may faintly respond to the test for endodermis. Leaf. Occasionally there may be seen secondary tracheides bordering the xylem of the leaf-trace at the base of the petiole. No cambium has been observed, there is merely an addition of single elements. Root. No observations regarding secondary thickening have been recorded. MARATTIA FRAXINEA. Stem. The process is of exactly the same nature as has been described for Angiopteris. There is also, in Marattia, the same thickening of the parenchyma bordering on the stele, but not to so great an extent as in Angiopteris. As regards the other members of the Marattiaceae, no addition of secondary elements seemingly takes place in Arcluiugiopteris, Dauaea or Kaulfussia.
On Secotidary Thickening in Pteridophyta. 211 EQUISETACE/E. Literature. Cormaek. Ona Cambial Development in Equisetum. Ann. Bot Vol. VII., 1893. EQUISETUM MAXIMUM. Stem. The examination of a transverse section through a mature node shows that the xylem of the bundles is relatively very extensive, and that the elements of the wood and of the phloem, in the central region of the strand, are arranged in radial rows. This regular seriation is not obvious in the tracheides nearer the centre of the axis nor in the more peripheral phloem-elements, in fact " the whole arrangement suggests that a plate of tissue has been intercalated between two older growths by the activity of a cambiumlike meristem." The opinion here expressed is strengthened by the examination of tangential and radial longitudinal sections, from which it is seen that the characters of the elements interposed between the xylem and phloem are those usually associated with a normal cambium ; and further, the formation of new cells is similar to that obtaining in ordinary secondary thickening. Corroboration is to be obtained by the comparison of sections of stems of different ages. By this means it is found that the number of elements in the radial thickness of a bundle is about equal both in a young and an old internode. Thus in this region the tangential division is arrested very early. This, however, is not the case at the nodes; the immature node has fewer elements in the radial thickness of a bundle than has the mature node. Cormack's conclusion is " that after the bundle has attained in the internode its full number of cells in radial thickness, and after tangential division in the corresponding tissues of the node has ceased, a plate of tissue has been intercalated between the protoxylem and protophloem of the node; tbat the xylem thus formed is mostly reticulately thickened, whereas the thickening of the protoxylem is annular ; and that the intercalation has been accomplished by the activity of a meristem whose cells are cambiform." PSILOTACE/E. Litcraluve. Boodle. Secondary Tracheides in Psilotum. New Plivtolocist Vol. III., 1904. ^ ^ ' Boodle On the Occurrence of Secondary Xylem in Psilotum, Ann. Bot., Vol. XVIII., 1904. Ford. The Anatomy of Psilotum triquctrum. XVIII., 1904. Ann. Bot., Vol
212 On Secondary Thickening in Pteridophyta. PSILOTUM TRIQUETRUM. Stetn. The formation of secondary xylem elements takes place in much the same manner as in Ophioglossuin and they occur in the subterranean and aerial axes. No definite cambium is present, but it is possible, in some cases, to make out a radial seriation in the tracheides and the adjoining parenchymatous cells. The new tracheides are sometimes pitted in an irregular manner, but more often the pits are of the normal scalariform type. The secondary xylem elements are situated between the primary wood and the phloem, and, generally, are separated from the former by at least one row of parenchymatous elements. On the other hand they may be in direct contact with the central xylem-mass, and in great part they replace the zone of parenchyma which, in younger axes, occurs between the primary xylem and the phloem. The examination of longitudinal sections demonstrates that these secondary tracheides pursue a sinuous course, which fact is probably due to sliding-growth. Boodle, who was the flrst to discover these tracheides in Psilotum, supposes that their formation depends on the development of the aerial shoots, " and appears to be due to a basipetal stimulus from the latter." TMESIPTERIS. So far as the published accounts go, Tmesipteris does not exhibit any secondary thickening ; it would not be at all surprising, however, to find secondary tracheides in the axes of this plant, provided material sufficiently old were examined. SELAGINELLACE/E. Literature. Bruchmann. Untcrsuchungen iiber Selaginella spinulosa. Cotha, 1897. SELAGINBLLA SPINULOSA. Stem. This is the only known species of Selaginella which shows the feature under consideration. At the base of the stem, which is a permanent structure, there is an indefinite growth in thickness. The phenomenon is very similar to what occurs in the same region oilsoetes Hystrix, but with this difference, that whereas in the last-named plant there is an addition of secondary parenchyma only, in the case of Selaginella spinulosa the later-formed elements consist of parenchyma and xylem.
On Secondary Thickening in Pteridophyta. 213 ISOETACEAE. Literature. Von Mohl. Ucber den Bfiu des Stammes von Isoetes lacustris. Vcrmischte Schriften. Linnaea, 1840. Hofmcistcr. Bcitragc zur Kcnntniss der Gefjiss-Kryptogamen. I. Die Entwiekelungsgeschichteder Isoetes lacustris. Abhand. der K. Siichs, Ges. d. Wissensch., IV., 1852. Russow. Vergleichende Untersuchungen, St. P6tersbourg, 1872. Hegelmaier. Zur Kenntniss einiger Lycopodien. Bot. Ztg, 1874. Farmer. On Isoetes lacustris. Ann. Bot., Vol. V., 1891. Smith, R. Wilson. The Structure and Development of the Sporophylls and Sporangia of Isoetes. Bot. Gaz., Vol. XXIX., 1900. Scott and Hill. The Structure of Isoetes Hystrix. Ann. Bot., Vol. XIV., 1900. ISOETES ECHINOSPORA, I ENOELMANNI, AND I. LACUSTRIS. Stem. The cambium arises comparatively early, about the year after germination in the case of /. lacustris, in the parenchymatous tissue surrounding the axial strand. The divisions extend around and above the central cylinder, but not so far upwards as the youngest leaf-trace. The products of the cambial activity are parenchyma towards the periphery and elements of the so-called prismatic zone towards the centre. The amount of secondary xylem produced is always very small, and sometimes may be quite absent, a fact not surprising in in these aquatic plants. ISOETES HVSTRIX. Steiit. As is well known the cambium in this and the other species examined is a well-marked tissue and arises very early in the life of the plant. There is a certain amount of variation in its position. In some cases it is situated in the tissue immediately towards the exterior of the primary xylem and between the wood and the phloem, in which case it may form new wood, the elements of which directly abut on the tracheides of the primary xylem. This activity however does not last very long, for a new merismatic zone arises further out towards the periphery of the stem. This new cambium forms secondary parenchyma towards the exterior while, on its inner side, it gives rise to parenchyma, phloem and a variable amount of xylem. The anomaly to be observed here is the formation of centrifugal phloem. This succession of cambial zones is not of general occurrence ; in the majority of instances the merismatic zone last considered is the only one produced, and it remains active throughout the whole period of secondary growth in thickness.
214 On Secondary Thickening in Pteridophyta. A few cases have been described in which there was produced a new cambium internal to the flrst. In these cases the new meristem arise in the parenchyma secondary in origin a little towards the exterior of the primary xylem. This cambium behaves in a precisely similar fashion to the pre-existing one, giving origin to new parenchyma, phloem and wood on its inner side. It is obvious that the secondary thickening is, in this plant, very variable. Taking its most constant features, as regards the locality of the cambium and the direction of formation of its secondary products, as the normal, it is more like the process obtaining in Dracaena, a fact pointed out by the earlier observers. The amount of secondary xylem produced is very variable; these tracheides are always present however and they agree in all essentials with the lignifled elements of the primary wood. There remains to be mentioned the slow downward growth of the stem, a feature which has been made much of in the past. This downward growth is due to the activity of the cambium, which at the base of the stem appears to form parenchyma only. This is a point of some importance and shows that there is no real homology with the apical growth of a primary root. It is not proposed to enter into any elaborate discussion regarding the significance of the presence of secondary thickening these plants. Attention, however, may be drawn to the opinion that, on the whole, the recent Pteridophyta exhibit considerable reduction. A large number of the fossil Pteridophyta possessed marked powers of secondary growth in thickness, and therefore it is not a matter for surprise to find that certain of their descendants still retain traces, more or less well-marked, of this capability. For example the obvious connexion between Equisetum and Catamites may be cited. For these reasons the secondary tissues in the recent forms are to be looked upon as instances of reduction rather than cases of the inception of new developments. T.G.H.