GROWTH PHASES IN THE LIFE OF A LICHEN THALLUS

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Dominick Ellis
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1 New Phytl. (974) 73, GROWTH PHSES N THE LFE OF LCHEN THLLUS BY R.. RMSTRONG The Btany Schl, Oxfrd University {Received 2 March 974) SUMMRY The grwth rates f thalli f flise saxiclus lichens befre and after the linear phase f grwth were measured in 973. Changes in the radial grwth rate (measured as mm/year) with thallus size in the prelinear phase (thalli less than apprximately.5 cm in diameter) were cnsistent with the hypthesis that early grwth f these lichens is lgarithmic. When grwth in the prelinear phase was measured as a relative grwth rate (measured as cm^/cm'^/year) there was a rapid rise in grwth rate until abut 3 mm thallus diameter and then a decline in grwth rate. The radial grwth rate f nn-fragmenting thalli when cmpared with fragmenting thalli at different stages f fragmentatin suggested that the radial grwth rate des nt significantly decline after fragmentatin f the thallus. This result is nt cnsistent with a pstlinear phase in the radial grwth f a lichen thalius. NTRODUCTON n a previus paper (rmstrng, 973) the measurement f grwth rates f thalli f fur flise saxiclus lichens (three Parmelia spp. and ne Physcia spp.) f diameters frm t 2 cm led t the cnclusin that the radial grwth rate f a thallus (expressed in mm/year units) was cnstant and the relative grwth rate (expressed in cm^/cm^/year units) declined expnentially after a thallus diameter f apprximately.5 cm was reached. thallus f these species which is greater than abut.5 cm in diameter grws with a cnstant increase in radius each year and this grwth phase has been called the linear phase. t has been suggested by previus authrs that there are tw mre grwth phases during the life f a lichen thallus. Firstly, there is a grwth phase which precedes the linear phase (the prelinear phase) where the radial grwth rate changes with time and is thught t be lgarithmic in frm (Platt and msler, 955; Beschel, 96). Secndly, there is a grwth phase which succeeds the linear phase (the pstlinear phase) where the cnstant radial grwth rate slws after fragmentatin f the central regin f the thallus (Hale, 967). This paper describes the grwth rates f small thalli (between and.25 cm) and thalli which have fragmenting centres because there are few data n bth the prelinear and pstlinear radial grwth phases. MTERLS ND METHODS Tw series f grwth rate measurements were carried ut n thalli in 973 at the site at the Muth f the River Dvey (Nat. Grid. Ref. SN 696) in Suth Merinethshire described by rmstrng (973). 93

2 94 R.. RMSTRONG The prelinear grwth phase T investigate the prelinear grwth phase, thalli f Parmelia cnspersa (Ehrh ex. ch.) ch. and P. glabratula ssp. fuliginsa (Fr. ex Duby.) Laund., with greatest diameters frm t.25 cm were cllected n small rck fragments f Ordvician slate in Octber and Nvember 972. Rck fragments with ne r tw small thalli were cllected s that grwth culd ccur withut interference frm neighburing thalli. Sixteen thalli f each species were cllected. The pstlinear grwth phase T investigate the pstlinear phase f grwth, thalli f P. glabratula ssp. fuliginsa with greatest diameters frm. t 6. cm were cllected n rck fragments f Ordvician slate in Octber and Nvember 972. Sixteen nn-fragmenting thalli f different diameters and twenty-fur fragmenting thalli at different stages f fragmentatin were cllected. ll the rck fragments were placed n hrizntal bards in an unshaded site in the field s that the thalli were grwn under similar cnditins. Measurement f grwth The grwth f the thallus was measured using the methd f rmstrng (973) where the mean radial grwth f a sample f lbes was measured under a magnificatin f XO. Between eight and ten lbes were measured fr each thallus except fr the very small thalli with few lbes where between three and five lbes were measured. The grwth f the thallus was measured at the beginning f each mnth fr the twelve mnths f 973. Grwth w^as measured n air-dry thalli in the labratry near t the sample site. The grwth rate f the thallus was expressed as a radial grwth rate (mm/year) and as a relative grwth rate (cm^/cm^/year) using the methd f Wlhuse (968). The yearly grwth rate f the thallus was btained by adding up the measurements fr each mnth in 973. Measurement f fragmentatin f the thallus s a result f fragmentatin a space is frmed in the centre f the thallus. t the end f the grwth experiment the shape f the thallus and the shape f the space in the centre f the thallus were drawn n tracing clth. The ttal area f the thallus and the area f the space were estimated by superimpsing the traces n graph paper. T measure percentage fragmentatin, the area f the space in the centre f the thallus was expressed as a percentage f the ttal area f the thallus plus space. RESULTS The relatinship between thallus diameter (cm) and grwth rate expressed in units f mm/year and cm^/cm^/year fr tw flise lichens, Parmelia cnspersa and P. glabratula ssp. fuliginsa, is shwn in Fig. i. There is an increase in the radial grwth rate with increasing thallus diameter and the data are cnsistent with the hypthesis f a prelinear radial grwth phase in which the thallus radius increases lgarithmically. When grwth rate is expressed as a relative grwth rate (cm^/cm^/year) there is evidence fr tw early grwth phases. There is a rapid rise in the relative grwth rate until a diameter f 2-3 mm, and then a decline in grwth rate. The relatinship between the radial grwth rate (mm/year) and thallus diameter (cm)

3 Lichen grwth O CM E u 4 2 _ E ~ a> 5.? 3 O X) cc D i_ 'wth cr Relat J ^ i. 2 - ~ J Thallus diameter (cm) Fig.. Grwth rate f small thalli f Parmelia cnspersa (circles) and P. glabratula ssp. fuliginsa (triangles). Filled symbls, radial grwth rate; pen symbls, relative grwth rate. fr bth the fragmenting and nn-fragmenting samples f P. glabratula ssp. fuliginsa is shwn in Fig. 2. The means f the radial grwth rates f thalli in the cnstant radial rate phase f the fragmenting and nn-fragmenting samples are nt significantly different by a ^-test at the P.5 level f prbability. This result suggests that the radial grwth. ) E _ 5 TJ D Ct Thallus diameter (cm) Fig. 2. Grwth rate f Parmelia glabratula ssp. fuliginsa., Nn-fragmenting thalli; O, fragmenting thalli.

4 R.. RMSTRONG ^ 3 a> E 5 \ > TO tr f J L J L Percentage thallus fragmentatin Fig. 3. Grwth rate i Parmelia glabratula ssp. fuliginsa at different stages f fragmentatin. rate f a nn-fragmenting thallus is nt significantly different frm that f a fragmenting thallus f equal size. Hwever, the spread f pints abut the mean is greater fr the fragmented sample than fr the nn-fragmented sample which suggests that a few fragmented thalli have a lwer grwth rate than a nn-fragmented thallus f equal size. The radial grwth rate f P. glabratula ssp. fuliginsa at different stages f fragmentatin f the thallus is shwn in Fig. 3. s the percentage fragmentatin f the thallus increases the radial grwth rate remains relatively cnstant. f the grwth f the fragmenting thalli was expressed as a relative grwth rate (cm^/cm^/year) then the beginning f the pstlinear phase wuld be marked by a rapid rise in the relative grwth rate. These results are nt cnsistent with the hypthesis f a marked decline in the radial grwth Prelinear phase Linear phase Frgmentatin f thallus - \' " / ^ Thallus diameter Fig. 4. Summary f changes in grwth rate f a thallus thrugh life and the crrespnding grwth phases: p, radial grwth g rate (mm/year); / y ) ;,, relative grwth g rate ( ^ ( / / ^ / year); ) ( ), thallus radius (cm), () (i) = i ncreasing radial dil grwth h rate in prelinear phase; (2) = cnstant radial grwth rate in linear phase; (3) = changes in relative grwth rate in prelinear phase; (4) = negative expnential decline in relative grwth rate in linear phase; (5) = increasing relative grwth rate after fragmentatin f the thallus; (6) = lgarithmic increase in radius in prelinear phase; (7) = cnstant increase in radius in linear phase.

5 Lichen grwth 97 rate and hence a pstlinear grwth phase where the radial grwth significantly slws after fragmentatin f the thallus. summary f the changes in the grwth rate f a flise thallus f the lichens examined thrugh life and the crrespnding grwth phases derived frm rmstrng (973) and the present wrk is shwn schematically in Fig. 4. DSCUSSON The results reprted in the present paper n tw flise lichens tgether with the results f rmstrng (973) frm wrk n fur flise lichens have led t the cnclusin that the linear phase f grwth where the radial grwth rate is cnstant is preceded by a prelinear phase which is apprximately lgarithmic in frm. n additin there is n evidence that the radial grwth rate significantly slws after fragmentatin f the thallus. lthugh Hale (967) has shwn that the radial grwth rate f Parmelia cnspersa declines after a thallus size f abut 3 cm is reached, the result reprted in the present paper is nt cnsistent with a pstlinear phase in the radial grwth f a lichen thallus. f grwth is expressed in relatin t the ttal area f the thallus (the relative grwth rate), the prelinear phase is marked by a rapid rise in grwth rate fllwed by a decline, the linear phase by a negative expnential decline and the nset f fragmentatin f the thallus by a rise in grwth rate. Fr the relative grwth rate expressed in relatin t the ttal area f the thallus t be a valid measure f grwth it has t be assumed that all carbn that is fixed by the thallus is ptentially available fr radial grwth. This assumptin presuppses that there is translcatin f fixed carbn frm the centre t the grwing edge f the thallus. Hwever, there are few data t supprt this assumptin and hence there is sme dubt f the validity f measuring relative grwth rate by expressing grwth in relatin t the ttal area f the thallus. n additin, the radial grwth rate remains relatively cnstant in large thalli after fragmentatin f the thallus. This result suggests that the centre f the thallus des nt cntribute t the radial grwth f large thalli. Hence, althugh a pstlinear phase in the relative grwth f a lichen is suggested by the data, the rise in the relative grwth rate after fragmentatin f the thallus is a result f expressing grwth in relatin t the ttal area f the thallus. mre accurate measurement f the relative grwth rate f large thalli culd be made by expressing grwth in relatin t the peripheral area f the thallus. This measurement wuld nt increase after fragmentatin f the centre f the thallus. Hence, the data are nt cnsistent with a pstlinear phase in the radial grwth r the relative grwth f a lichen thallus. The delimitatin f the grwth phases is based n grwth rate measurements f thalli f different size s that each stage in the life f the thallus is represented by a different thallus. This methd f determining the grwth phases f a thallus thrugh life is nt ideal because the variability in radial grwth rate between thalli f equal size when grwn under similar cnditins is high (rmstrng, 974). This variability will bscure the differences in grwth rate between thalli which are due t the age f the thallus. The delimitatin f the grwth phases is als based n measurements f grwth rate in ne year (972 fr the linear phase and 973 fr the prelinear phase). There is evidence frm a preliminary study n tw successive years' grwth t suggest that the yearly variatin in grwth rate f a thallus is high (R.. rmstrng, unpublished) and this variability will bscure differences in grwth rate which are due t the age f the thallus. n spite f these surces f errr in the methd the changes in grwth rate which ccur with thallus size are large enugh nt t be bscured by the variability in the material. n

6 98 R.. RMSTRONG alternative apprach t the study f the grwth phases f a lichen n an undated substratum culd be made by measuring the grwth rate f a single thallus ver several successive years but such a lng-term study is beynd the scpe f the present investigatin. The radial grwth f the thallus has been cnsidered in rmstrng (973) and in the present study and has been expressed as a radial grwth rate (mm/year) r as a relative grwth rate (cm^/cm^/year). This measurement, which is relatively easy t make fr a lichen thallus, is nly ne parameter f grwth. There may be changes in the thickness f the thallus with age as well as increases in area which will cntribute t the ttal dry matter prductin f the thallus. t sme stages in thallus develpment the cntributin t the ttal dry matter prductin f the thallus by an increase in thickness may be greater than the cntributin frm radial grwth. diagram f the grwth phases f a thallus which is based n changes in thallus thickness as well as radius may be significantly different frm the scheme suggested by the present study. Cnsequently a study f the changes in thallus thickness which ccur with time is needed. CKNOWLEDGMENTS The help f Dr D. C. Smith in discussins f this wrk and in the preparatin f the manuscript is gratefully acknwledged. This wrk was financed by a N.E.R.C. Research Studentship. REFERENCES RMSTRONG, R.. (973). Seasnal grwth and grwth rate clny size relatinships in six species f saxiclus lichens. New Phytl., 72, 23. RMSTRONG, R.. (974). The structure and dynamics f saxiclus lichen cmmunities. D.Phil, thesis, University f Oxfrd. BESCHEL, R. (96). Dating rck surfaces by lichen grwth and its applicatins t glacilgy and physigraphy (lichenmetry). n; Gelgy f the rctic, Vl. 2, p. 44. University f Trnt Press, Trnt. HLE, M. E. (967). The Bilgy f Lichens. Edward rnld, Lndn. PLTT, R. B. & MSLER, F. P. (955). basic methd fr the immediate study f lichen grwth rates and successin. J. Tenn. cad. Sci., 3, 77. WLHOUSE, H. W. (968). The measurement f grwth rates in lichens. Lichetilgist, 4, 32.

DATING LUNAR SURFACE FEATURES BY USING CRATER FREQUENCIES* T. J. Kreiter

DATING LUNAR SURFACE FEATURES BY USING CRATER FREQUENCIES* T. J. Kreiter Divisin f Space Sciences Jet Prpulsin Labratry Califrnia Institute f Technlgy INTRODUCTION Several theries have been advanced relating