Wood anatomy of insular species of Plantago

B U L L E T I N
OF
T H E
T O B E E Y
VOL. 97, No. 6, pp. 353-361
B O T A N I C A L
C L U B
NOVEMBER-DECEMBER
W o o d anatomy of insular species of Plantago
p r o b l e m of raylessness
1970
a n d the
Sherwin Carlquist 1
Claremont Graduate Sehool and Baneho Santa Ana Botanic Garden,
Claremont, California 91711
CARLQUIST, SHERWIN (Claremont Graduate School, Claremont, California 91711).
Wood anatomy of insular species of Plantago and the problem of insular woodiness. Bull.
Torrey Bot. Club 97: 353-361. 1970.—The most markedly woody species of Plantago are
insular. Detailed anatomical descriptions, quantitative data, and photomicrographs are
presented for woods of P. arborescens (Canary Islands, lowlands), P. maderensis (Madeira, lowlands), P. webbii (Canary Islands, alpme areas), P. fernandeziana (Juan Fernandez Islands, rain forests), and P. princeps (Hawaiian Islands, shady areas in rain
forest). Vessel element length and width are greatest in the rain-forest species, least in
the alpine species. These expressions seem clearly correlated with degrees of xeromorphy
or mesomorphy. Plantago woods are all basically rayless, but rays are eventually formed
in larger stems of the species from Macaronesia (the term given collectively to the Cape
Verde Islands, Canary Islands, Madeira, Azores, and nearby islands). Rayless woods as
exemplified bj' Plantago tend to occur in dicotyledons in which (1) eambial activity is
limited or finite; (2) woodiness is phyletieallv in the process of increase, rarlier tha
crease; (3) fusiform eambial initials are relatively short; and (4) juvenilism or paedomorphosis occurs.
The phenomenon of ^insular moodiness," the tendency" for insular representatives of a taxonomic group to be more
woody than their continental counterparts,
is well illustrated by Plantago. This worldwide genus is herbaceous, yet on various
islands of the world, woody species are present. These have evolved, at least in part,
from several groups of herbaceous mainland plantagos. The Pacific insular species,
certain!}-, have been derived independently
of those from the Maearonesian (Atlantic
islands. To be sure, no Plantago is markedly
woody, but the insular species do have more
secondary xylem accumulation than their
closest mainland counterparts.
I'hi at ago has arrived on many islands,
presumably by means of its characteristic
dispersal mechanism. Seeds are small and,
when mature and moistened by rain or
otherwise, develop a gelatinous film that
fastens them to feathers or other animal
parts. Plantago also seems to establish well
on islands because of its weedy tendencies.
I was surprised to find that in my garden
in Claremont, a Canary Island endemic, P.
arborescens, became naturalized despite the
fact that the climate of Claremont is more
continental than that of the Canary Islands.
These escaped plants are facultative annuals in Claremont: they germinate after
autumn rains, grow into shrubs during winter and spring, but then die during the
summer. They are, however, able to repro-
duce during this period. In the Canaries.
P. arbort«c< ns can live through the mild
and more humid summers and thus last for
several years. In fact, in the relatively
maritime climate of Santa Barbara, California, cultivated plants of P. arbor•<m
also do this.
Plantago arborescens is one of a series
of closely related Maearonesian species. It
occurs near sea level to about 400 m in the
Canary Islands, and can be found with
shrubby species of EcJiium and Sonchus
and other elements of the coastal scrub,
such as Hypericum, Aeonium, and leguminous shrubs. Plantago arborescens does not
grow in the wet laurel forest or in the
pifiar. Contrary to its name, it is not really
tree-like: a large specimen rarely is more
than half a meter in height. The stem, at
the base, rarely exceeds 1-2 cm in diameter.
Plantago webbii is an alpine counterpart to
P. arborescens, almost an alpine ecotype.
It grows in the high, dry caldera of El
Teide (ca. 3,000 m), Tenerife, CanaryIslands. Here it may be found with shrubby
crucifers, Spartocytisus, and other alpine
plants. For ecological data and illustrations
of the Canary Island plantagos, the reader
may consult Burchard (1929), Lems (1960)
and Schenck (1907).
1
Field and laboratory studies have been aided
by two grants from the National Science Foundation, GB-4977X and GB-14092.
Received for publication July 3, 1970.
3o-)
354
BULLETIN OF THE TORREY BOTANICAL CLUB
Plant ago maderensis of Madeira occurs
Dear sea level on Madeira, with
Euphorbia.
Aeonium,
Lytantkus,
a n d shrubby crueifers. I t is closely related to P. arborescens
both in morphology a n d ecology, a n d may
I"' regarded as a vicarious species of the P.
arborescens g r o u p .
riant ago princeps is a distinctive Hawaiian endemic. Unlike other Hawaiian
plantagos, it has slender u p r i g h t stems with
relatively long internodes. In habit it recalls some species of Dracaena. Xow extremely rare, Plantago princeps once was
a b u n d a n t in ravines, on cliffs, and near
waterfalls in the H a w a i i a n Islands. The
stems, although u p r i g h t , lack mechanical
strength a n d the p l a n t s tend to lean on surr o u n d i n g shrubs, or to grow up through
such other plants. The wood sample used
in the present s t u d y was collected by Mr.
Melvern Tessene in the Aina IIo Valley,
Koolau Gap, Maui. This plant would fall
into P. princeps var. la&ifolia according to
the t r e a t m e n t of Rock (1920). Rock recognizes numerous varieties of P.
princeps.
These might be termed subspecies accordin-.'' to c u r r e n t concepts. I prefer, however.
to think of P. princeps as a single polymorphic species. In fact, P. princeps is a p p a r ently fully interfertile with Hawaiian
plantagos of an entirely different appearance, including bog species such as P.
pachyphyUa, P. hillebrandii. etc. (Melvern
Tessene. persona] communication). Ecologieal data a n d photographs of P. princeps
a n d other H a w a i i a n plantagos are offered
by Kock (1920) a n d Carl.piist (1970a).
Plantago fernandeeiana
is a species of
rain forest in the J u a n F e r n a n d e z Islands.
Ms ecological situation is much like t h a t of
P. princeps.
The internodes are shorter
than in P. princeps. but it also tends to
form u n b r a n c h e d u p r i g h t stems topped by
leaf rosettes. Good photographs of P. fertinndi :inIKI a n d ecological information are
ided by Skottsberg (19S2). One cann o t be positive t h a t P. fernamleziana
is not
closely related to P. princeps, b u t it could
AVCII be a n insular derivative from mainland plantagos independent of other insular
Pacific species of Planiago, In the Atlantic.
the distinctive P. rob'uata, an endemic of
St. Helena, is almost certainly n o t closely
related to the Maearonesian plantagos. F o r
information a n d illustrations of P. robusta.
the reader is referred to Melliss (1875) and
Carlqnist (1965). Unfortunately, no wood
[VOL.
97
of this species, which may now be extinct,
was available to me.
The significance of Plantago woods lies
in their response by means of increased
woodiness to the uniformity of insular climates. These plantagos are, in fact, sensitively adjusted to the ecological conditions
in which they grow. They are not, in m y
opinion, relicts: they are all on volcanic
islands of modest age. A p p a r e n t l y phyletic
increase in vessel-element length to suit
mesic conditions can take p l a c e ; if so, the
mechanism is a p e r t i n e n t subject for invest igation.
Plantago woods are all rayless, at least
earlier in the ontogeny of a p l a n t . The significance of rayless woods in relation to
habit, ecology, level of phyletic specialization, direction of phylesis, a n d n a t u r e of
wood ontogeny, needs to be understood.
Data of the present p a p e r are p e r t i n e n t to
this problem.
Wood of Plantago princeps was cited
by the writer (1962) in such a context. A
description of wood of Plantago, based on
P. fernandeziana,
was offered b v Metcalfe
a n d Chalk (1950).
M a t e r i a l s a n d m e t h o d s . Woods of the
Maearonesian species of Plantago were collected by the writer in the field. The largest
plants available were taken for study. Some
samples were unsuitable for study, however, because of markedly distorted wood
grain—stunted, twisted stems are common
in the Maearonesian species. The writer is
indebted to Mr. Melvern Tessene for the
sample of P. princeps. The samples of P.
fernandeziana
were provided t h r o u g h the
assistance of Dr. Otto Solbrig, D r . F r e d
Meyer, and Dr. Richard H . E y d e .
Wood sections a n d macerations were
p r e p a r e d according to the usual techniques.
The wood samples available were reasonable
in size for a wood study, a n d samples with
too limited a development of secondary
x vie in were deliberately omitted. F o r example, liquid-preserved stems of P. pachyjdndla could have been sectioned by means
of paraffin techniques, b u t the limited development of secondary xylem in this speeies would, in m y opinion, not have added
significant information, at least with respect to the central concerns of the p r e s e n t
study.
Qualitative and quantitative d a t a were
obtained from six collections of the five
species studied here. I n rayless or near-
L9T0
CARLQUIST: WOOD ANATOMY OF PLANTAGO
rayless woods, ray measurements are not
of significance. The diameter of libriform
fibers was obtained by measuring the widest
point of liber diameter as seen in a maceration and averaging 50 such measurements.
A figure for vessels per group was not computed because groupings in /'.
fmiiindeziana
Fi& 3) are almost infinite; vessels
in P. webbii are often as narrow as libriform fibei's and therefore difficult to identify in a transection. For <|iialitarivc data.
50 or more measurements per Feature p e r
collection were employed. T a b u l a r summarization, as used in ray earlier papers in
wood anatomy, is u n f o r t u n a t e l y n o t feasible because so few species a r e involved.
Anatomical descriptions. PLANTAGO
C E N S P o i r . , C a r l q u i s t 2 4 3 6 ( R S A ) , B a r r a n c o do
San Andres, Tcnerifo, Canary Islands ( F i g s . 5 - 6 ) :
V e s s e l s 1 8 - 2 2 ( a v e r a g e : 8 7 ) u la d i a m e t e r . Vessel
its 9 8 - 3 0 2 (»ver;i•_'.•: 171) JJ. long. V e s s e l s i n
r a d i a l Chains, s..!it;«r_\ vessels also c o m m o n . P e r
f o r a t i o n p l a t e s simple. Pita On vessel walls a b o u t
4 JA in diameter. Pits circular, alternate, a very
few elliptical pita also observed. Libriform fibers
132-311 (average: - i ' i p in length; diameter at
widest point 9-27 (average: 22) p.. Libriform fibers often slightly wider radially than tangentially.
Wall thickness of libriform fibers about 3 p. Bays
if, or a few remnants of exceptionally wide
p r i m a r y r a y s present
U a very few rays in;
tardily in larger stems. A few fibriform cells present adjacent to vessels may perhaps be termed
axial parenchyma cells by virtue
ft
which
Save
wider a p e r t u r e s tlian t y p i c a l f o r lit .ri form
Q r o w t b r i n g s p r e s e n t ; e a r l y wood c h a r a c terized by w i d e r vessels a n d wider- l i b r i f o r m libers.
which are also slightly thinner than those "f late
wood. A few or some libriform fibers storied. No
deposita of resin-like compounds observed.
PLANTAGO ABBOBESCENS, Oarlqnist 2725. from
S a n t a C m * de I.a I ' a i m a . I .a P n h n a , C a n a r y I s l a n d s , s i m i l a r in q i i a n t i t a t
res t o t h e a b o v e ,
has t h e following :
Or q u a n t i t a t i v e c h a r i t i e s : rassel d i a m e t e r , 33 u : vessel-element
l e n g t h , 17.! u ; libriform-fiber lengtS 208 u : libriform-fiber w i d t h , 2 9 p..
P L A N T A G O vm
D \N'A B e r t . , F . G. M e y e r
9645 ( U S ) , M a s a t i e r r a , J u a n F e r n a n d e z I s l a n d s
( F i g s . 1, 2 , 1 2 ) : Vessels 21-5-1 a v e r a g e : 4 0 ) n
;
in dinim •
9 2 0 2 - 3 0 7 (avei
2 6 4 ) n long. V e s s e l s in r a d i a l c h a i n s of Indefinite
l e n g t h , a f e w s o l i t a r y vessels also p r e s e n t . P e r f o r a t i o n p l a t e s s i m p l e . Pita e n vessel walls a l t e r n a t e , a b o u t 5 p, i n d i a m e t e r , c i r c u l a r . L i b r i f o r m
fibers 271—607 ( a v e r a g e : 4 4 5 ) p in l e n g t h ;
e t e r at widest point 1 5 - 4 0 ( a v e r a g e : 29) p. Ldbrif o r m fibers m o s t l y much w i d e r r a d i a l l y t h a n t a n g e n t i a l l y . Wall t h i c k n e s s of H b r i f o r m fibers a b o u t
3 p.. R a y s a b s e n t . A few f i h r i f o n n cells present ad-
355
j a c e n t t o vessels m a y p e r h a p s be termed axial
p a r e n c h y m a cells by v i r t u e of t h e i r p i t s , which
ider a p e r t u r e s t h a n t y p i c a l f o r l i b r i f o r m
fibers. A very few such cells s u b d i v i d e d i n t o s t r a n d s
of S cells were o b s e r v e d . G r o w t h r i n g s a b s e n t , differences from o n e p o r t i o n of xyloin t o a n o t h e r t o o
m i n o r to p e r m i t d i s t i n c t i o n a s t r u e g r o w t h r i n g s .
No s t o r i e d s t r u c t u r e o b s e r v e d . N o r e s i n - l i k e d e posits present.
P L A N T A G O M A D E B E N S I S D e c n e . , C a r l q u i s t 2622
( B 8 A ) , Ribeiro B r a v a , Madeira ( F i g s . 7 - 8 ) : Vessels 2 1 - 5 2 ( a v e r a g e : 3 2 ) p. i n d i a m e t e r . Vessel elem e n t s 91-178 ( a v e r a g e : 1 4 1 ) ^ l o n g . Vessels in
radial chains, b u t m a n y solitary. Perforation plates
simple. L a t e r a l wall p i t t i n g a l t e r n a t e , p i t s circular, a b o u t 4 u. in d i a m e t e r . L i b r i f o r m fibers 135—
259 ( a v e r a g e : 2 2 4 ) u. i n l e n g t h ; d i a m e t e r a t widest.
p o i n t 1 5 - 2 6 ( a v e r a g e : 2 1 ) p . L i b r i f o r m fibers n o t
iciably wider r a d i a l l y t h a n t a n g e n t i a l l y . Wall
t h i c k n e s s of l i b r i f o r m fibers a b o u t 2.."> p.. A few
rays p r e s e n t in o u t e r p o r t i o n s of l a r g e r wood s a m ples. Rays c o n s i s t of e r e c t a n d s q u a r e cells. Pibri
f o r m cells, s o m e s u b d i v i d e d i n t o s t r a n d s of 2, a d j a c e n t t o v e s s e l s ; t h e s e seem c l e a r l y i d e n t i f i a b l e
as p a r e n c h y m a cells. G r o w t h r i n g s p r e s e n t : e a r l y
wood d i s t i n g u i s h e d b y s l i g h t l y wider vessels a n d
l i b r i f o r m fibers. S o m e l i b r i f o r m fibers s t o r i e d . \ n
d e p o s i t s of resin-like m a t e r i a l s o b s e r v e d .
P L A N T A G O P B I N C E P S Cham., collected in 1965 b y
.Melvern T e s s e n e ( M I C H ) , A i n a H o V a l l e y , K o o l a u
Gap, M a u i , H a w a i i a n I s l a n d s ( F i g s . 1-2, 1 3 ) . Vessel d i a m e t e r 3 1 - 6 0 ( a v e r a g e : 3 8 ) p.. Vessel ele151—291 ( a v e r a g e : 2 4 6 ) n l o n g . Vessels
m o s t l y s o l i t a r y , b u t a f e w in s h o r t r a d i a l c h a i n s .
Perforation plates simple. P i t s o n lateral walls of
vessels a l t e r n a t e , p i t s c i r c u l a r , a b o u t i ji in d i a m
et&c, L i b r i f o r m fibers 1 1 0 - 2 5 1 ( a v e r a g e : 187) p.
in l e n g t h ; d i a m e t e r a t widest p o i n t L1-2S ( a v e r
a g e : L ' p . L i b r i f o r m fibers n o t a p p r e c i a b l y wiiler
r a d i a l l y t h a n t a n g e n t i a l l y . W a l l t h i c k n e s s of fibers
3 i u. Bays a b s e n t , b u t b e g i n n i n g of r a y f o r m a
tion s u g g e s t e d by o c c u r r e n c e of s h o r t e r fibers in
w h a t would b e i n t e r f a s c i c u l a r a r e a s . G r o w t h r i n g s
as such not p r e s e n t , a l t h o u g h a f e w n a r r o w e r r i n g s
of thin walled fibers p r e s e n t ( b o t t o m of F i g . 1 ) .
S o m e l i b r i f o r m fibers s t o r i e d . N o d e p o s i t a of resinlike m a t e r i a l s p r e s e n t .
P L A N T A G O W E B B I I B a r n . , C a r l q u i s t 2494 ( R S A ) ,
C a l d e r a of E l T e i d e , T e n e r i f e , C a n a r y I s l a n d s
( F i g s . 9 - 1 1 ) : Vessel d i a m e t e r 7 - 3 0 ( a v e r a g e : 17)
l«. Vessel e l e m e n t s 72—170 ( a v e r a g e : 1 2 8 ) p. long.
Vessels in s h o r t r a d i a l c h a i n s o r s o l i t a r y . A few
viTV n a r r o w vessels w i t h o u t p e r f o r a t i o n p l a t e s , a n d
t h e r e f o r e v a s c u l a r t r a c h e i d s , b u t n e a r l y a l l vessels
with p e r f o r a t i o n p l a t e s . P e r f o r a t i o n p l a t e s a l l s i m ple. P i t s on l a t e r a l w a l l s of vessels a l t e r n a t e , circ u l a r , a b o u t 4 ^ i n d i a m e t e r . L i b r i f o r m fibers 1 1 0 219 ( a v e r a g e : 1 8 7 ) p, i n l e n g t h ; d i a m e t e r af widest p o i n t 8 - 1 6 ( a v e r a g e : 1 3 ) p . W a l l t h i c k n e s s o f
l i b r i f o r m fibers a b o u t 2 p . L i b r i f o r m fibers n o t
appreciably wider radially t h a n tangentially. Bays
a b s e n t in c e n t e r of s t e m ; p r e s e n t i n o u t e r p o r t i o n s
of l a r g e r s t e m s . R a y cells s q u a r e t o e r e c t . A few
356
BULLETIN OP THE TORREY BOTANICAL CLUB
Figs. 1 4 . Sections of wood of Pacific species of Plantago.—Fig.
1-2. Plantago princeps, coll. M.
Tessene. Pig. l. Transection, Slowing zone of thin-walled libers below.—Fig. 2. Tangential section.
Some of the libriform fibers show :i storied condition.—Figs. 3-4. Plantago fernandeziana,
F . Meyer
9645. — Fig. 8. Transection. Vessels are arranged in conspicuous radial chains.—Fig. 4. Tangential section. Tracheary elements are notably long for Plantago. The micrometer scale above F i g . 1 was photograph
e same scale as Figs. I 1": it allows L35
i subdivided into intervals of 10 u each.
1970]
CABLQUIST: WOOD ANATOMY OF PLANTAGO
357
Pigs. B B. Beetions of wood of Macaronesian species of FtaxtOffO.—Figs. 6 6. Flantago arborescens,
Caxlquisl 2436.—Fig. 5. Transection. A growth ring is visible near the top of the photograph.—Fig. 6.
Tangential section. A storied pattern may be seen in some of the lihriform fibers.—Figs. 7-8. Plantago
», Carlquist 2622.—Fig. 7. Transection. Most vessels are solitary.—Fig. 8. Tangential section.
Beginning of ray formation may be seen near the center of the photograph. Scale of magnification above
Pig. l applies to Figs. 5-8.
358
lin.LETIN OF THE TORREY BOTANICAL CLUB
l V O L . •••'.
. 9-13. Wood sections of Plantago.—Figs. 9-11. Plantago webbU, Carlqnist ~2±9i.—Fit'. 9. Tranion. Several growth rings may be seen; deposits of unidentified materials appear dark.—Fig. 10.
Tangential section. Note narrowness of vessels.—Fig. 11. Portion of tangential section at higher magnification, showing a ray, left.—Fig. 12. Plantago /'.-niaicl' ~iana, Meyer 9645. Portion of tangential section. Compare cell size to thai of P. webbii. Fig. 11.—Fig. 13. Plantago princeps, coll. M. Tessene. Transection, showing margin of pith at left. Interfascicular areas are suddenly converted to fibers during
commencement of secondary growth.—Figs. 9-10. Magnification shown by scale above Fig. 1.—Figs. 1 1 -
ile of magnification shown by scale above Kg. 13.
I!l70j
CARLQUIST: WOOD ANATOMY OF PLANTAGO
(Uniform veils present adjacent to vessels may perhaps be termed axial parenchyma cells by virtue
of their pits, which have wider apertures than
typical libriform fibers. Growth rings present, characterized by wider vessels and libriform fibers in
early wood. A few libriform fibers storied. Deposits
of resin-like materials present in many cells, particularly in central portions of stems.
Ecological correlations. The n a t u r e of
vessel-element dimensions with relation to
ecology was examined for Asteraeeae by
the writer (1966). Vessel elements are
shorter a n d wider in xeromorphie species
in this family. \Yh<-re a taxonomic g r o u p
immigrates to a n island a n d a d a p t s from
d r i e r to wetter conditions, increase in vessel-element diameter a n d length occurs
d u r i n g this evolution. This is t r u e in u n r e lated groups of dicotyledons: Campanulaceae tribe Lobelioideae (Carlquist, 1970b),
Scaevola of the Goodeniaceae (Carlquist,
1970c), Euphorbia
(1970d) a n d Echium of
the Boraginaceae (1970e). These t r e n d s
might, also be expected, therefore, in Plantago. I n fact, this proves to be t r u e :
Species
P. webbii
P. maderensis
P. arborescens
P. princeps
P. fernand-eziana
Vessel
diameter
(average, n)
17
32
33
38
40
Vessel-element
length
(average, p,)
128
141
173
246
264
L e n g t h of libriform fibers parallels this
t r e n d . One would expect this, for length
of fusiform cambial initials governs length
of both libriform fibers a n d vessel elements.
One might not have expected vessel diameter to parallel length of t r a c h e a r y elements so perfectly, but evidently these respond to the same ecological factors. The
distinctions in the table above m a y n o t
seem very great, b u t the range of differences is p e r h a p s compressed owing to the
fact t h a t in this specialized genus length
of t r a c h e a r y elements is close to the minim u m for dicotyledons.
Diameter of vessels is wider in early
wood of ring-porous dicotyledons a t large,
so t h a t one would n a t u r a l l y expect wider
vessels to be an indication of mesomorphy
and, therefore, dicotyledons of mesic situations to have, on the average, wider vessels t h a n those of xeric regions. This a p pears to be true, on the basis of Asteraeeae
(Carlquist, 1966). I n all likelihood, wider
vessels are not a phenotypic modification
but a genetically-controlled a d a p t a t i o n to
359
mesomorphy, a n d the reverse applies to
xeromorphy.
The d a t a of the present s t u d y do not
answer whether the longer vessel elements
of P. fernandeziana
( F i g s . 4, 12) a n d P.
princeps ( F i g . 2) are the p r o d u c t of paedomorphosis, although t h a t explanation seems
very likely (Carlquist, 1962). If length of
fusiform cambial initials decreases in woods
showing juvenilism, then shorter length in
the Macaronesian species m i g h t be expected ; however, s t u d y of radial sections of
these species suggests t h a t they begin with
shorter elements t h a n do the woods of P.
femancU nana and P. princeps. I feel t h a t
increase, phylogenetically, of length of
fusiform cambial initials may be possible,
at least within certain l i m i t s ; this possibility was suggested earlier (Carlquist,
1966).
The accumulation of the resin-like materials in wood of P. webbii m a y be a response
to the xeric conditions in which it grows,
for such accumulations seem most common
in woods of desert plants, stem suceulents
excepted. Because there is such g r e a t ecological similarity between the h a b i t a t of
P. fernandeziana
a n d t h a t of P.
princeps,
the curious difference in vessel g r o u p i n g is
difficult to explain. This m a y be evidence
that these two species are not closely related but independent derivatives from
other plantagos. Too often, the similarities
in growth form are taken as evidence of
phylogenetic relationship, a n d insular rosette-trees a n d rosette shrubs tend to be
misunderstood on this account.
R a y l e s s n e s s . All woods of
Plantago
begin with a rayless condition as soon as
production of secondary xylem commences.
As shown in Fig. 13, interfascicular areas
a iv converted to fibers r a t h e r suddenly.
Rays, if produced in species with rayless
woods, occur only in outer portions of the
secondary xylem, as noted by B a r g h o o r n
(1941) for Geranium tridens. Such r a y s
are shown here for P. webbii ( F i g . 11) ;
rhey a p p e a r to be produced in the Plantago
species with more extensive accumulation
of secondary xylem. The fact t h a t Plantago
is both woodier on islands a n d also rayless
is i n t e r e s t i n g ; one m i g h t not think this
u n u s u a l except t h a t the p a t t e r n is repeated
in other insular dicotyledons. Other dicotyledonous groups t h a t are basically
herbaceous a n d show both increased woodi-
360
B U L L E T I N OP T H E TORREY BOTANICAL CLUB
nrss am! raylessness in the Hawaiian flora
include Geranium. (Barghoorn, 1941), Lysiimirlii't and Viola (Carlquist, u n p u b l i s h e d ) .
In New Zealand, Alseuosmia
is rayless
(Barghoorn. 1 9 4 1 ; Paliwal and Strivaslava. 1969). M a n y dicotyledons in insular
situations are not actually rayless but show
a condition very similar morphologically:
predominance of erect r a y cells, often with
absence of procumbent cells (Carlquist,
li'Tub, 1970c, 1970d).
A number of rayless woods have now
been reported. Some of these are listed by
Barghoorn (1941). Others have been given
by Boureau (1957). F r o m my own experienee, I can a d d Jaeobima cirnea
(Acanthaceae) and Leptodachflon
californicum
• I'olemoniaeeae). The n u m b e r of rayless
woods now known is great enough so t h a t
commentary concerning raylessness seems
warranted. A l t h o u g h no class of dicotyledons is predominantly rayless and. indeed,
only a t i n y proportion of dicotyledonous
woods are rayless, raylessness appears to
occur in the following classes of dicotyledons :
(1) Groups in which cambial activity
is limited or finite. One g r o u p of examples
is offered by genera siu-h as
I'luntago,
Viola, etc., which are herbaceous with very
little secondary xylem except where special
conditions, as the uniform climate of certain sub-tropical islands, p e r m i t a slight
increase in production of secondary xylem.
Anomalous secondary growth is probably
result, in at least sum.' --roups, of loss
of normal cambial activity d u r i n g evolution toward an herbaceous mode of structure. If a single cambium can no longer
produce more t h a n a finite a m o u n t of vascular tissue, then successive cambia can produce a larger stem or root diameter. T h a t
anomalous woods are often rayless was
noted by Barghoorn (1941) a n d has also
reaffirmed in the ease of
Bougainrilha by E s a u a n d C h e a d k .'1969).
(2) Groups in which woodiness is in
the process of increase, rather than decrease. The s t a r t i n g point is an herbaceous
condition, not a woody one. This situation
is represented on insular and tropical or
subtropical areas where uniform climatic
conditions permit a more nearly indefinite
growth season and. therefore, a more n e a r l y
indefinite accumulation of secondary xylem.
A group of dicotyledons which immigrates
to -inch uniform situations a n d a d a p t s to
[VOL.
1»7
the year-long growing season might conform to this situation, a n d virtually all of
the known rayless genera might be said to
have these specifications.
(3) Groups in which fusiform cambial
initials are relatively short. These are, of
course, many of the same groups that a r e
herbaceous, for herbaceous groups do tend
to have, in general, specialized wood featores of which short fusiform cambial
initials is one. I n a g r o u p with k m g fusiform eambial initials, the disparity between
the length of fusiform initials and t h a t of
ray initials is g r e a t ; the difference musl
!)•- little or none for raylessness to occur.
(4) Groups in which a form of juvenilism or paedomorphosis occurs. As specified
in the .in
point, a group in which fusiform and ray initials are similar in length
is needed for the occurrence of a rayless
wood. This situation would be altered, however, and raylessness lost, if subdivision of
ray initials occurred. This actually happens in some rayless woods that ultimately
form rays, e.g., Plantogo m hbii or Geranium Iridi ns. One may, of course, ask wh\
formation of rays should be forestalled or
delayed. Do plants of an herbaceous a n
t r y or with relatively little secondary xylem
aeciimulation have normal xylem function
without rays, and is ray parenchyma necessary for the normal function of large
masses of secondary xylem? There is presumably a selective value in the preset
and s t r u c t u r e of rays.
The above considerations do not totally
explain ra
3, obviously. For example.
no explanation is offered why the cambium
should add thick-walled cells (libriform
fibers) to interfascicular areas r a t h e r than
thin-walled cells (parenchyma cells) as
secondary growth begins ( F i g . 13). The
n a t u r e of phloem in species lacking rays
also has been neglected a n d needs inv
gation.
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