In ' P awe r a ' red c love r seed , a grea t va r i e ty o f c e l l types occur a t the hilar and s ub - h i l ar r e g i on s a n d the t i ssues of these regions
a r e extreme ly comp lex . Th'-' shape and p o s i t i on of the hilum is
1 43
s imila r to t h a t f ound i n some o t her t y p e s of le gume seeds ( Corner 1 9 5 1 ; Hi l l e r 1 9 6 7 ; Chm-1dhury and Huth 1 9 7 0 ) , although the h i lar s inus i s no t o cc luded b y the radicular lobe as f o u n d in s ome s pe cies e . g . Crotalaria
CHill e r 1 9 6 7 ) . Ther e f o r e , t h e h i l um can be seen c learly when the f uniculus i s d e t ached f rom the seed. l n g e ner a l , ce l l s t ruc ture and the arrangemen t o f t he t i s s ue s a t t he h i Lu1n i s mo re o r l e s s t he same as d e s c r ibed in
the s e e d s o f Cr•o ta laria s p e c it:! S (�1 i l J L' r 1 9 6 7 ) and Lupinus species
(Hyde 1 9 5 4 ) .
The s hape o f the t rache i d bar ;md surrounding c e l l s a l so shows s imil arity to corre s J,ond i n g s c r u c t u r L: s in s owe o t he r legumes . S ince the
t racheid t is sue i s c o nne c t e d v: i t h t h L! v ascu lar b undle there seems t o be s ome func tional r e l a t i onsh i p b e t w L!en these s tructures. Thi s r e la tionship may be impor tan t p a r t i c u l a r l y i n t he e a r l y deve lopme n ta l s tages o f the s eed wh ich depend on the s up p l y ll f nu t r h: n t s from the parent p lan t . The emb r y o may po s s i b l y ob t a in n u t d v n t s n o t onl y f rom the va s cu la r
s upp l i es b u t a l s o t h ro u gh t L e t ra d 1 e Ld tissues s ince t h e la t te r o c c u p i e s mo s t o f the sub-hilar r e g ion .
The dev e lopmen t and d i f f e r e n t i a t i o n o f p a l i sade c e l ls and tracheid tis s ue a t t he hilar re g i on o c c u r s d u r i ng the ear l y s tages o f .seed
develo, ::le n t as rep o rL, d by c,J cn<.:r ( 1 Y 5 1 ) . The present f in d ings a l so
s how ::;ome a g r eeme n t \v i t h t ll L: \vO J"k o r �1i l l e r ( 1 9 6 7 ) and Amb egaoka r ( 1 9 7 6 ) o n the s i z e and s hape o f p a l i s u d e c e l l s and w i t h Mi l le r ( 1 9 6 7 ) and Cho\vd h u r y and B u t h ( 1 <:17 0 ) u t i c h e a r rangeme n t o f t he s e ce l l s a t t h e h i.lum. T h e hilar reg ion o r ' l'mve ra ' n�J c l ove r seed c on t a i n s the typical cha rac t e r i s t i cs o f s e e J s o f memb e r s o[ t h e P a p i l iona c ea e .
In ' Pawer a ' red c l o v e r , a d a r ke r c o l o u r e d l ine a p p e a r s a s t h e
c u t ic l e o f t he test a is ob s e rv e d , 1 y j ng be twe e n t h e p a l i . , .Jde a n d c o un t e r p al isade ce l l s a t t h e h i l um . A s i m i l a r e f fec t h a s b e e n r e p o r t ed b y Hy de ( 1 9 5 4 ) i n d i p lo i d r �d d n v l:! r . HO\vcve r , t-l i l l L:I ( 1 9 6 7 ) r e p o r ted
that t h e r e is no c u t i c le t 11l :,..: Ldy e r s o f c e l l ::; i. n c b L� s e e d o f
1 4-4
Usually sclerenchyma tous tis sues a t the sub-epidermal region do no t conta in any obvious intercell ular space s . However , as s tated by Miller ( 1 9 6 7 ) , intercellular spaces occur at the inner por tion o f the
t i s sue which is i nt egra ted w i t h t he parenchyma of the sub-h i lar region . The pos i t ion of the mi cropyle in the seed of ' Pawera ' red c lover i s s imilar to tha t found in Crota laria seed (Miller 1 9 6 7 ) and i s
probably the same in some other Trifo lium species . Although , the micro p y lar pore is open in mos t seeds the mi cropy lar o r i f ice,appears to be b locked in some seeds . The micropyle , t herefore may be b locked or may rema in open as repor ted by Esau ( 1 9 6 5 ) and Fahn ( 1 9 7 4 ) . In s eed sec tions , s ince the micropy le appears as an open canal , observa tions in the present s tudy do no t agree wi th reports on the s truc ture of the micropyle in Phaseo lus and Vicia (Pres t on and Scott 1 94 3 ) . Moreover ,
in no cases were t he pal isade cells f ound to be c losely uni ted about the mi cropy le as found by Mar t in and Hat t ( 1 9 4 4 ) . I f the micropylar
end i s no t b locked , the mic ropylar canal appears as an open canal in s eed sect ions . However , t he inter ior end of the canal t ermina tes before reaching the embryo .
The pos i t ion and shap e o f the s trophiole in 1 Pa;vera ' red c lover is s imila r t o tha t f ound in Me l i l otus a lba and 1'r-ifo Zium subterraneum
(Hamly 1 9 3 2 ; Hagon and Ba l la r d 1 9 7 0 ) . As repor ted by Miller ( 1 9 6 7 )
w i t L seeds o f Crota laria� some var ia t i ons i n the degree o f conf igurat ion was noted w i thin seed samp l e s . S imi lar l y , co lour var ia t ion was a
cons tan t fac tor . There fore , ne i ther of these aspe c t s appear to be reliably useful for seed iden t i f i ca t ion as previously sugges ted by
Miller ( 1 9 6 7 ) . In longimedian sec t ion t he configura t ion of the s t roph iole
can be called doub ly convex a s termed by Miller . In ma ture seeds , the p ropor t ion of the he ight o f the ma lp igh ian c e l l s a t the s t rophiole compa red with correspondi t,M c e l l s in the lateral wa 1 1 shows a s imilar arrangement to tha t wh i c h occurs in rf'r>ij'o l ium subter•raneum (Aitken
1 9 39 ) . Howeve r , even i n young ovules of ' Pawera ' red clove r, d i f f erences in the he ight o f t he ma l p ighian c e l l s u t the s trophiole comp ared wi th
the ·ells of the la te ra l wa l l is not aR large as in Plswn sutivum (Rc�uve 1 9 4 6b ) .
1 45 The arrangement o f t he cells a t the p lane o f symme try i s s imi lar to tha t f ound in sub terr anean clover (Hagon and Ballard 1 9 7 0) . The p resence of the underlying s c lerenchyma cells at the s t rophiole seems
to be a charac ter i s t i c o f Trifo l ium spec ies . Mor eover , as found in
Me li lotus a lba (Hamly 1 9 3 2 ) and Tr•ifo lium subterraneum (Hagon
and Ballard 1 9 70 ) , the long a nd narrow malp i gh ian cells of t he central region of the p lane s eem to show def inite cap s when comp ared to their adj acent cells .
The p resence o f the compac t cell layer de tec ted immediB:tely beneath t he malp igh ian cells o f t he central plane in the s trophiole o f ' Pawera ' red c lover seed has not been rep o rted by previous workers . As this s truc ture seems to appear only a t cer tain s tages o f seed developmen t , i t may not be possib le to iden t i fy this s truc ture in mature seed . Mo reover , thi s s t ruc ture can only be seen in transmedian sec t ion .
The vas cular sys tem in the seed o f ' Pawera ' red c lover s eems to b e s imilar to tha t found in some o ther legume seeds (Martin and Wa t t 1 9 44 ; Esau 1 9 60 ; Fahn 1 9 7 4 ) , al though there may have some var iat ion in different species , genera , or sub -fami l ie s (Corner 1 9 5 1 ) . In the p resen t s tudy no recur ren t and post-chala zal vas cular bund les were noted
as reported by Corner ( 1 95 1 ) . Howeve r , the reports of Corner ( 1 95 1 )
and Esau ( 1 9 6 0 ) agree with the p resen t f ind ing s that the va scular
bundlH s tops a t the chaluzal r egion with shor t branches ex tend ing t o the s ide of the seed . The vascular sys t em j_n the seed of ' Pawena ' r ed
c lover is the re fore a lBo ve ry � im i lu r to tha t de s cr ibed by Mi l ler ( 1 9 6 7 ) in CY'o ta laria .
1 46
( b ) Seed coa t s t ruc ture and i t s re l a t ions to
LITERATURE REVI EW
The longevi ty o f v iab le seeds and their ab i l i ty or o therwise to
germina te .ilien p laced under cond i t i ons sui tab le for growth i s s trongly condi t
ioned by the ab i l i ty o f seed to imb ibe w� te r . The seed coat o f mos t mat ure legume seeds i s hard , re s i s tant to ab ras ion , covered w i th a
wax-Like layer and imp eremab le to wa ter . Thi s phenomenon i s mos t widespread in the Leguminosae and some o ther fami l ies such as Cannaceae , Convolvulaceae , Ge raniaceae , L i l iaceae , and Malvaceae ( Harring ton 1 9 1 6 ; Coe and Mar t in
1 9 2 2 ; Wa tson 1 9 48 ; Anon 1 9 5 2 ; Cracker and Barton , 1 9 5 3 ; Ballard 1 9 7 3) . The seed coats in several gene ra o f the Leguminosae have been
inves t i gated extens ive ly to de t e rmine the s t r u c ture concerned w i t h the cause o f impe rmeab i l i ty (Pamme l 1 89 9 ; Whi t e 1 9 08 ; Hamly 1 9 3 2 ; Ai tken 1 9 39 ; Mar t in a nd Wa t t 1 9 4 4 ; Watson 1 948 ; Anon 1 9 5 2 ; Hyde 1 9 54 ; Bal lard 1 9 7 3 ; McKee e t a l . 1 9 7 7 ) . I t has been agreed b y mos t workers (Mar t in and Wat t
1 94 4 ; Reeve 1 9 4 6a ; Corner 1 9 5 1 ; Hyde 1 9 54 ; Esau 1 9 6 0 ; Quinl ivan 1 9 7 l a • Ball ard 1 9 7 3 ; McKee e t a l . 1 9 7 7 ) tha t the outer-mo s t c e l l layer o f the seed coa t , known as the mal p i g h ian or pali sade l ayer i s responsible for the
p revent ion of wa ter en try i n to t he seed. Howeve r , a s the seed coat is rather complex , cons i s t ing o f a number of s t r uc t u r 12 s , opin ions are also d iv ided
ove r the p a r t icular structures invo l veJ .
As early as 1 8 7 6 , Nobbe (c i t e d b y Harton 1 9 65 b) sugge s ted tha t imper meab i l i ty is due to a waxy la yer ov er t he seeds. Some o ther early worke rs
(Pamme l 1 8 9 9 ; \vh i t e 1 9 0 7) repo rted that the cut i cle of the seed coa t is
impermeab le to wa t e r . They rec orded t ha t t he d e g r e e o f .i mp e rme a b i Li ty is i ncreased w i t h the t h i ckening o f the cu t i c le . Rees ( 1 9 1 1 ) suppo r t ed their
f ind l.ngs tha t the seed coa t s of soml' s pec i e s o f Ind'igo.fer·a� Cytisus� and
Ac:acia are more imp e rmeab l e b e c a u s e of t h e pre sence o f a thicker cuticle . She also s ugges ted tha t i mp e rmeab i l i ty i n Acacia loph2ntl� and [�nna indica� i s due tn c u t i c u l ar i z ed th i,:ken L ng i n t h e ma l p igh .ian c e l l s .
Mar t i n und Ha t t: (l 9 L1 4 ) rl' IH> r t u d thn t th�' c u t i c u l n r i :.-!l.'d l i.lye r , domes and l igh t l ine co nstitute a ba r r i e r to water i n dry r i pe seeds o f swe e t c love r . To rrey ( 1 9 6 8 ) gener<�ll y s ug ge s t e d tha t in some s e e d s i mp e rmea b i l i ty
to water or oxygen o r to both may be due to waxy or impervious layers in the seed coa t .
1 47
Gu tterrnan and Heydecker ( 19 7 3 ) found that seeds o f Ononis sieuZa ( Le guminosae) wh ich have mat ured on the parent p lan t during long days take a longer t ime to imbibe water than thos e ma tured in shor t day s . They p u t forwar d a p roposal tha t cuticular thickn�ss o f the s ee d coat i s respon s ib le f o r the delay in imb ib i t ion oc curring in long day seed s . Smar t t ( 19 7 6 ) , working wi th some tropical pulse s , desc ribed the presence o f a wa ter-proof
imperv ious layer in the tes ta and a waxy depos i t over the h il um and micropyle p r even t ing the movement of wa ter into the seed .
Watson ( 19 48 ) , however concluded that in the twenty-one species of legumes he examine d , variation in the thickne s s of the cuticle was ex tremely sma l l and could not be linked with the degree of impermeab i l i ty of the seed .
Rees ( 19 1 1 ) exp lained tha t in the seed of Me Zi lotus a lbus� the sub c ut icular layer may act as a cemen t subs tance to hold the cuticularized ends of the cells together and cause impermeab i l i ty . S imilarly , a report by Hamly { 1 9 32 ) cla imed tha t the impermeab le region o f the seed coa t of
Me li lotua alba i s £on11ed by a layer of t i ghtly appressed suberin caps