ON THE SEEDLING STRUCTURE O F TILIA VIYLGARIR. 329

On the Seedling Structure of lIFZia w l p i * i . s Heyne. By H. S. HOLDEN, L).Sc., F.L.S., and S. H. CLARKE, B.Sc., University College, Nottingham.

(\Vith 20 Text-figures.)

[Rend 19th November, 1926.3

ALTaouaH Tilia vztZga?*is flowers freely in England it is apparently uncommon for it to set any appreciable number of good seed. A few, however, are produced each year by a small group of trees in the vicinity of University College, Nottingham, as a careful search usually yields on0 or two seed- lings *. Tlie spring and summer of 1923 seem to have been exceptionally f:ivourable for the ripening of the seed, ns iii the spring of 1924 upwards of seventy seedlings were collected ; it is upon those tliat the present account is based.

MORPHOLOGY. Tlie strncture and germination of the seed and the morphology of the

seedling have been described and figureJ by Lubbock (9), and figures of the seedling are also given by Marshall Ward (16) and by Kerner (8). The cotyledons are epigeal and thinly pubescent, and are typically five-lobed. We inay conveniently designate the lobes as the median, intermediate, and lateral lobes respectively. I t is a point of some interest to note that there is quite a considerable variation in the proportion of the lobes one to another and in ~riinor features also. Among the specimens collected a t Nottingham the coinmonebt lype, which inay be referred to as the mean type, is shown in fig. 1. I t will be noted that the median and lateral lobes are some- what larger thau the t w o intermediate ones and that all are somewhat obtusely pointed. F r o m such a mean type a graded series of variants may be derived, two extremes of which are illustrated in figures 2 and 3. I n the first of these the variation consists in the elongation of the lobes which have consequently bocoine more acute, whilst in the second the whole cotyledon has undergone compression so that the lobes are relatively short and stumpy. Occasionally some degree of asymmetry is produced by inequalities in the lobing. One such case is shown in fig. 4, in which the intermediate lobe on the right is much sinaller than its fellow, whilst in a second case (fig. 5 ) there appears to have been a partial fusion of the apical lobe and the left intermediate lobe. This second specimen is also interesting 0 1 1 account of the lobes having rounded apices instead of the more normal pointed ones. I n addition to the variation in the size of the main lobes the development of subsidiary lobes both on the median and lateral Iobes may be noted. Fig. 6 shows their symmetrical development on both, M hilst in fig. 7 an example of

* Specimens from these trees have been deposited in the Botany Department of the Britieh Museum (Natural History).

thcir developnrent on the criitral IoI)e oii1j is sliown. ~1 i t .y mij. I)c little: more than diglit ontgrowtlls of the loI )e -~~~: i i~gin :is ill i i i c :isyiiinict~~it~:il v a ~ s11ow11 in tig. 8 ((, or be st,rongly tlevthlopetl ; I S shown in fig. ! I , Tliere is 110

indication of co-ordination hetween tlre two cotylctlons of tlre s ~ i i i e see(1ling in tlie clereloptiient of t,hc a c c ~ s o r p lol)cs, thoiiglr \ v I i ~ ~ r e thvy ~ I Y > well dcvelopeit i n one they are x t least represent,etl iii the otlior. sirmll accessory lobe o n one lateral lolle is shown in Lri1,l)ovl;'s figiirc (9, fig. 2!?3), whilst they are well developed on :i!l t,hc 1:ttor:iI 1 0 1 ~ : s of t,hc sceilling figured by Marshnll Wart1 (16, fig. 159). Kcrner's figure shows a slight 011~

on one of t!ie median lobes only (8, fig. 1-48>. As frequently happens w l ~ c n a large Peries of wetlliligs is collectetl, a few

spc in iens exhibiting some degree of abnormality were obtained ; these inclu~led hot11 syncotyls and poljcotj ls . Of the latter, one (fig. 10) was

FIGS. 1-12.

I

Varintioiis i i i tlie uiorphology of the cotjledon5.

amphi~yncotylous and showed supprerzion ot' an interniet1i:ite lobe in one cotyledon, whilst two others (figs. 11, 1 2 j s h o w d uiii1:iter:il s p c o t j IJ which had resulted in the reduction of the lohw on tlie sjniphpsis side. The polycotyls werc two in nuinber, namely it Iiemitricotyl and a tricotj 1: the cotyledonary members of the former hping 5honii i n figs. 8 a, S b.

HIs'roLoGT.

The surface hairs clotliing the seedling :iw of two kinds and sliow well- marked differences in distribution. Most of' them are uiiicellular and end in an acute point (fig. 13), though occasionally a single transverse wall may I)e present, The hairs of this tyt)e are scxtteretl o \ e r the I~ypocotyl, but in the cotyletlons they are confined to the areas ovcr tbe vcins and to the petioles. Each arises as an epidermal outgron tli, hut those on tlic upper surf:ice differ from those on the lower surface in h i n g snrronndcd hy cellz which form a slightly raised area (fig. 14). The base of the hair in nian) cases appears to

P'1c.s. 13-16.

I1 Fig. I :;. A pointed nnicellolar hair from the lover epiclermiv of tlie cotyledon ; tlie majority

;ire considt~rahly loiiper tlian tlir one fipiired. Fig. 14. Base of a Imir from the iipper epiderniis ,if the cotyledon, to sliow tlie larger epidermal cells a d the sliglitly raised area siirroundiii~ the h i r . Pig. 15. Elongated base of a hair in surface view. Fig. l ( j a, h, c. Club-shaped hair in longitudinal and tiaiisverse sections. The cell- contents of the hairs, wliicli are similar in all, are oiily sliowii in tig. 13.

The cells O E botli types of hair are crowded with briglit, yellow glolules of all sizes (fig. 13). These have tlie appenrance of oil-drop, hut d o not give tlie reactions c11:tracteristic of fatty substnnces. They are in~oliiblc! in alcoliol, chloroforni, ether, aiid xylol ant1 a re unaffected Iiy either acids or a1k:tIis. \Ire have been unable t,o deteriuiiie their precisc nat,ure, but their resistance to the action of solrents ix a point of some interest,.

Tho cells of the upper epidermis are polygoiial i n surface view, those over tlie veins being compressed 1:tter:tlly. ' l l i c ~ are distinctly larger than those of tlie lower epiderniis (qt: figs. 13 & 14) . The walls of the latter a re sinuons in character (fig. 1 7 a ) esceilt for those below t,he vein?, which resemble thc cell5 sirrii1:wly situxtcd oii ilic. i1pi)er surfare. The stoniatn a r c coiifiiietl to the under surface a.nd are reniarlwblc for their rariatioii in size. Two cliief'

332 DR. 13. 6 . IIOLDEN A S D XR. 9. € I . CLARKE OX

types are recognisablr, the first of which is relatiyell- large and is ovoid in ijlan, whilst the second is much hiiialler and is almost circular in plan (fig. 17 u) Among the stn;iller type a few cxaniples of twinned stomata haye been observed, one of which is ~ l i o w n in fig. 1 7 b.

Scattered tlirougli tho cortex of the cotyledonary petioles and of the hypo- cotyl : l id roots :ire cells containing nincilage, these standing out clearly after staining.

Fra. l i .

17b

pig. 17 a. l’ortioll of the lower epidermis froru the cotyledon, t o sllom the variation in the size of the stomettl. Fig. 17 b. The atime showing twinned stomatti.

The yellow fat-like globules occurring in the epidermal hairs, to which referQnce has already been made, are abundant in the bundle-sheath and the atljaceiit layers of the hypocotyl, petiole, and root, and are also present in the iiiedullary parenchynm. They are rare or absent in the cortical cells.

THE VASCULAR SYSTEM. The midrib of the cotyledon is a collateral bundle which becomes someivliat

extended tangentially following its junction with the bundles from the inter- mediate lobes. This increase in width becomes niore pronounced ns a result of the incoming of the strands supplying the lateral lobes. The union of the laterals witti the midrib may occur a t or near the base of the lamina or at various levels in the petiole. I n the composite bundle thus produced, although the phloem fornis a contiauous mass, i t is possible in Foung seedlings to differentiate the midrib xylem froni that of the 1ater;il lobes (figs. 18, 19, 20), although this becomes impossible in oltler seedlings. The characteristic triad structure, with its central file of xylem elements flanked by separate phloem groups, is not shown in the petioles of even the youngest seedlings examined, the phloem being continuous. A very frequent feature in the upper part of the hypocotyl is one in which the nietaxylem, as seen in

?'HE SEEDLING STRTJCTURE O F TILIA VULGARIS. 333

transverse section, forms two fan-shaped niwses, one on either side of the protoxylem. Occasionally the tnetaxylem elements nearest the protoxylem ciirve over this and, in the younger seedlings, as a rcsult, isolated metaxyleiii vessels may be seen abovc the protoxylem in the cotyledonary I h i e (fig. 20).

FIQS. 18-20.

18 _ _ - - _ _ .- -

\ \

20' Fig. 18. Transverse section of the cotyledonary midrib after union with the lateral strands,

Fig. 19. The same tit the top of the hypocotyl. Fig. 20. The same in the nliddle of the hppocotyl. Note the isolated metaxylem element ()I) above the disorganising protoxyleni in the Cotyledonary plane. Xylem only shown, phloem indicated by plain dotted line, junction of lateral atrnnds and midrib indicated t l ~ u s - . -, - . -

The epicotyledonary strands play no significant part in transition. As is the case in the Sycamore (Acer Pseudoplatanus) (13) the behaviour varies, the types noted being as follows :-

1. The phloem bifurcates and unites with that from the cotyledons, whilst the xylem dies out in situ.

2 . Both ])hloeni : ind \,yleni tlivitle :iii(l imitc \ri t l i tlie corrc>-pon(liiig

3. TIIP str<Lii(l niove, :I\ n wliole to iiiiite \I i t l i the c.otyldon:iry str;ind. I lie trnniition t o tyl)ic:il tcitr.ircli I oot-structnre i- e\trciiiir.l! lci3iirctly.

t he lateral xylcinz +el) irate from tliow of tlic niiclril) (fig. 201 I I I O I Y and niore witlrly and filially ntiite in the lower half 01' the hypocotyl. Here they clwelop group- of protoxyIt~ni and form typical wc:ints ill t h e ii1tc.r- cotyledonary plane. Actual root-structnre is only attaiiied w i i i u tli\taiicc IjrIon the collfjt.

Altliough i l i e wiueiice described abo\ A holdt good for 1 1 1 ~ ni:ijority of tlie ieetllings ex:iininctl R nunibcr of interesting variation< hai e hcen encountered. Tlir. simplest of these, and on0 which does not in any w:iy affect the root- striictiire, is produced by the coinplete indcpendeiice of' the lateral veiiis wliich a t no part of their course come into coiit:ict \\it11 the niiilrib huntlles. [hi, type of tr:insition, which occars only i n one seedling, is t~'pic:d of 11iany other 5pecies iiich, for eanniple, :ti h / m ~ k m Roylui (10, 11).

second variant, aI50 reprthen tcd hy olle seed iing only, 1i:iq rwul ted froln a coldition directly opliosite to that tlescrihed above. I n this second caie the Isteral strniitls of on(' si(le i n each cotylctlon I J ~ X O I I I ~ c~onil)lctely nicrged with the midribs and do not again itlpur,tte. The result is that Iriarchy obtains in the root, two of the pole.; being derixed froin tlie inidribs nrrtl One f rom the two laterals Tlie ti ansition ~)lienoniena in this seedling are rcininizcerit of those described by (hinptoii (3) for ,%wy~+ni t~~~ , hi~sutcc~n, allowance being niacle for the fact that tlie latter is a species ~ i t h independent lateral strands.

s t i l l a third variant, in this case represcnted by two seedlings, is one in whicli the root is pentarch. In one of thesv sredlings the earlier part of the transition is quite normal. Following the conil)lete separation of the laterals, however, the two on one side, instead of uniting, continue don n thc hypocotyl as independent collateral strands. Thebe gradunlly flatten out to fornl tarigeritiaIly extended plates and, just above the collet, develop separate protoxylenis in an exarch position. The phloeiiis bifurcate j u s t prior to the 2lppe:Lr:ince ot' the new protoxylems, and forin typical gtoups by lateral union with adjacent groups. The second example is one in which the whole of tlie tritiijition to root-structure is perfectly normal, SO that the root is primarily tetinrcli. I n the lower portion of the root, howeyer, a sinall group of ineta- xyleill vessels beconics separated off from one of the intercotyledonary pQ]es arlc1 moves laterally. The phloem group which overarches i t then tIii ides ;L[id exarch protoxylyn is developed opposite the gap thus formed, b~

prodllciiig a prntnrcli condition. I t beenis reasonable to regard this secoiitl e\:Lniple a s one in which the independencc of the lateral< a,< polc-foi.iiiing onits is :t>serted relatively late in thc transition, so that itb difference fro,ll the first example is one of degreo rather than kind.

elenient\ from tlie cot) ledons.

r i

r ,

A sinall n ~ d d l : t i? prwent tlirouglioiit tlie root.

7 1

liicli beh:i\ e normally.

THE SEEDLING SYRCCTVRE O F TILIA VVLGARIY. 335

Fi t t i regnrtl to the abnornial seedlings, only one of the i! ncotgls showed any vascukir ahuorniaiity, this being the one in which the unilateral syncotylj

AJ a re-ult of coinpression the vascular strands supplying the lateral 1 0 l ~ t . i on the symphysis side unite n ith the initlt ibs precociously. Illis union i- per~nnnerit so that only the two midribs and the second pair of latrralG forrn pole>, the root its :i consequence being trinrch.

The heriiitricotyl (fig. 8 cc, 6) and tricotyl were both pentarch, but in these seeding? the peutarchy has a totally different origin from that occurring in thc rlicotyl described in an earlier portion of this paper. I n both cases the midrib strands each formed a root-pole, the two remaining poles being pro- duced hy the lateral strmcls. These in one cotyledon beliaved quite normally, separating from the compounri central strand and uniting with similar laterals from the adjacent cotyledons, thus producing two further poles. I n the tricotj-l the suppression is due to the complete merging of one of the laterals in each of two cotyledons with the midrib. I n the hemitricotyl the adjacent lateral lobe3 along the line of junction of the two imperfectly separated cotyledons are not developed and, as a consequence, the lateral strands which \vould normally ,upply them are non-existent.

:%$ 1 n 0 h t I I I ~ I ~ W ~ .

r 1

DISCUSSION. Although a considerable body of work on seedling anatomy has now

accnmulated it has not yielded the clearly defined information regarding phylogeny which i t was hoped would accrue. All the same a number of interesting results have been obtained, and i t is reasonable to assume that with the continued pursuit of seedlin~,investigatio~~ a satisfactory conception of the features which characterized the ancestral types will be possible. Dr. Miles Thomas has already sumnlarimd certain aspects of her extended period of work in a recently published report (15), and her fuller account will be looked for with considerable eagerness. One fact which does emerge more or less clearly is that the variations in seedling anatomy :ire due not to any great number of fundamentally difl'erent basic types, but rather to a ringing of the changes on the components of a common group of vascular strands. The constituents of this common group, the chief of which are the cotyledonary midrib and its laterals, lllay undergo, in different seedlings, various changes with regard to linkage, fusion or independelice, and i n relative importance or position. Where, as sometimes happens, a new factor such as the epicotyledonary strands plays an important part a further series of variations is rendered possible (7).

Undoubtedly the commonest type of vascular arrangement in tetrarch seedlings is that in which the midrib and its laterals remain independent. The opposite condition in which they constitute a compact median compound strand is much loss frequent. It has been recorded and briefly described by Miles Tlionias (14) for AZtItcua, and by De Fraine for Opuntia and Nopaleu (6).

LINN. JOUR?J.-BOPANY, VOL. XLVIl. 2 E

Tilic~ furnizlir- 'ti1 e\niul)lr of an iiitcrnie,li,tte coutlition since, a!thongh the fusion of thc plilocni s! stein of tlie niedi:tn and lateral strands is coriiplete, the niiion of tlie ayleni strands is rc1,itiiel~ 10o.c~ and i5 rather in the nature OF :I c los~ linkage than a merging of itlentitp. The favt that 'lilin nay ediibit a type of transition in which tlict later,ilz reninin intiependent througll- out is worthv of notc in this connection. Siicli :I condition i i re1:itii ely r ~ i e in f i l i a , but other specie, sucli as P!/mrs wtnmu& (14) sliow :I large degree of variability. TiZin qlioiv5 some :tl)proach to Y p s C O ) I Z ~ W L ~ ' s a150 in the fact tliat the laterals Inn. :jbsIinie a more inqmrtaitt role in root-pole form- ation, each forming :L pole intlependently instead of uniting to form a. corninon pole. In Ililiu the condition is unilateral so that pentarchy results, whilst in Pyms both sets o f laterals may be inrolved, thus leading to n hesarch root condition.

Mi-s Bexon, who has recently made an intensi\e ctudy of AIfiwa (I), lras shown that in this case also a considerable amount of variation occurs pro- ducing a tetrarch-hexarch range in the root, so that in the case of that genus a more extended comparison with P p u s is possible.

The r~ r -e i se condition, in which a reduction from tetrarchy to triarchy occurs, is freqnennt in the Legurninosze, and Compton (3) interprets this a i th some degree of probability as illustrating a tendency towards reduction to diarchy froni an ancestral tetrarch condition.

It is remarkable that, in ?ilia, such :t reduction niay occur in what appears to be an absolutely norinal seedling, and this renders one less confident in ascribing to compression the loss of a pole in the syncotyl described earlier. A t the same time there is a considerable body of evidence (4,10,11,) that the compression consequent on unilateral syncotyly does lead to a lessening of the importance of the lateral strands on the symphysis side, and it niay be that in Tilia a rarely expressed potential reduction is rendered more frequent by such a compression.

'CVe are perhaps on -afer ground in ascribing the suppression of one of the intercotyledonary poles of the tricotyl to crowding, as it is cliaracteristic of the majority of polycotyls to find the number of poles reduced from what one would expect by coinp~irison with the norinal seedling.

I n an earlier paper on polycotyly (12) it was suggested that the division of the nieristeni which resulted in the hemitricotylous and other polycotj lous states might be either qualitative or quantitative. If the former, the midribs of the two resultant cotyledons would behave in transition like a single normal midrib ; if the latter, then each of the daughter midribs would show triad structure and exhibit a cartain degree of independence in transition. If this conception is sound, the hemitricotyl and tricotyl of Tilia described above both have resulted froni an apical division of tlie quantitative type.

*

SL%\T.iit B.

1. The seedling of 7’iliic r i d ~ / u ~ ~ i s po>sesses two typic:illy fibe-lohed epigenl

2 . The cotyletloixiry lob(+ vary in h e ant1 may show elongation or shortening corn1)ared with a coiiiiiioii mean type. They may also dewlop arcessory lobe3.

3. The aerial part5 are thinly pubwceiit, the hairs present being of tit o type- : - (a) Unicellnl:tr oiiec confinetl to the epidermal ridges ahove and below the veins of the cotyletlon lainins but gcnerally distributed over the hypocotyl. ( b ) Club4taped nirdticellolar hairs confined to the upper surfaces of the cotyledons between the veins. 4. The vascalar sydmn i g tetrarch in plan and typical root-structure is

on1-j attained 3 o i m di-Lance below the colleb. 5. Seedling3 showing triarchy or pentarchy also occur. 6. Syncotylou3 seedlinp, where the syncotyly is unilateral and at all

’7. The polycotylous seedlings exanlined are peatarch, though the peutarchy

cotylcdon~.

pronounced, shorn triarch ,symmetry.

occurring in this caqe is not honiologous with that occurring in dicotyls.

BIBLIOGRAPHY. 1. BESON, D.

2. COXI’TON, 1;. €I.

3. --

4, -- 5. CHAUVEAUD, G.

6. L)E FRA~NE, E. 7 . DATEY, A J. 8, KERNER, A. Natural I11storJ’’of Plants. English Editjo~l, vol, i. p. 623, fig, 148,

9. LUBBOCK, Sir J. On Seedlings. Vol. i. pp. 281-283, fig. 223. London, 1892. The Anatomy of \ome Atypical Seedlings of h p a t i e i i s Roylei.

An Anatomical Study of the Tariatiun in the Transition Phenomena in

Theories of the Anatomical Transitioo from Root to Stem. Xew

Journ. Linn.

Ann. 13ot. xxvii. 1913. Ann. Sci. Nat.

the Seedling of Alt?ma rosea.

Phgt. xi. 1912.

Soc. (Bot.), xli. 1912.

Ann. Bot. xl. 1926.

An Investigntion of the Seedling Structure in the Legumiiiosa.

An ,4natomical Study of Syncotyly and Schizocotyly. L’appareil conducteur des plnntes vasculaires etc.

Bot. ser. ix. t. xiii. 1911. The Seedling Structure of certain Cactacee. Ann. Bot. xxiv. 1910.

Seedling Anatomy of certain A2mentifere. Ann. Bot. xxx. 1916.

London, 1894.

10. HOLDEN, H. S. Ann. Bot. xxxiv. 1920.

11. I I o L D m , IT. Y., nud 31. 1)AxrRr.s. Further Studies on the Anatomy of Atypical Seedlings of Imputiens Roylpi.

12. HOLDEN, H. S., and 1). BEXON. The Anatomy of soine l’olycotglous Seedlings of Cheiraizthuu Cheiri.

13. ___ -- On the Seedling Structure of Ace,. Pseudoplatanus. Ann. Bot. xxxvii. 1923.

14. THOXAS, E. N. Ann, Bot. rxviii. 1914.

15. THOMAS, E. N. XILES. The Primary Vascular System in Phanerogams. Sectional Tmne. K, British Association, Toronto, 1924.

16. WARD, €1. KARSHALL. Trees. Vol. r. p. 279, fig. 159. Cambridge, 1909.

Ann. Bot. xxxv. 1921.

.inn. Rot. xxxii. 1018.

Seedling Anatonly of Ranales, Rhceadales, and Rosales.