glandular structures of hologarpha and their

glandular structures of hologarpha and their

Reprinted from the AMERICAN JOURNAL OF BOTANY. Vol 46, No. 4, 300-308. April, 1959
Printed in I'. S. A.
GLANDULAR STRUCTURES OF HOLOGARPHA AND THEIR ONTOGENY
SHERWIN C A R L Q I I S T
GLANDULAR STRUCTURES OF HOLOCARPHA AND THEIR ONTOGENY1
SHERWIN CARLQUIST
ABSTRACT
CAKKH I - I , NIEKWIN. (Rancho Santa Ana Botanic Garden, Claremont. California.) Glandular
structures of Holocarpha and their ontogeny. Amer. Jour. Bot. 4 6 ( 4 ) : 300-308. Illus. 1959.—
Two types of advanced glandular structures occur in the 4 species of the genus Holocarpha. Sessile
disk-shaped glands occur at the tips of upper leaves and of involucral and receptacular bract-.
Unlike all other glandular -irtictures of Madinae which have been investigated, these originate
from several protodermal initials rather than a single one. These glands, however, represent
modifications of a glandular trie-home. The other type of glandular structure, termed hollowi.iIked irirlinme here, occurs on the outer surface of involucral and receptacular bracts. These
trichomes originate from a single cell but differ from others in the formation of a hollow stalk.
I he wall of which is one cell in thickness. Mesophyll of the bract, often with an included vascular
bundle, is present as an intrusion into the base of the hollow stalk. Corresponding to the advanced
nature of the glandular structures, the leaves show specializations in the "innilling" of margin-.
I |i|»r leaves have a cylindrical organization of vascular tissue, whereas basal leaves are "normal"
and leave- of the main -inn are intermediate. The species of Holocarpha differ in certain details
ol leaf anatomy and structure of hollow-stalked trichomes. The systematic distribution of these
i- given. The essential unity of the various glandular structures of Madinae is discussed both in
terms ,,[ mature structure and ontogeny, and the steps in the evolution of these are suggested.
THE GENUS Holocarpha
(Compositae. subtribe
M a d i n a e ) consists of 4 species ( M u n z and Keck,
in p r o s i which a r e notable a m o n g the t a r w e e d s
b.r their higfi d e g r e e of specialization.
T h i s is
reflected in their c h r o m o s o m e n u m b e r s I J o h a n s e n ,
I').">.•}: Clausen. 1 9 5 1 1 , which d o not a p p e a r to b e
basic within M a d i n a e . as well a s in the c h r o m o s o m e
r e p a t t e r n i n g which has evidently been extensive in
the g e n u s I Clausen. 1 9 5 1 ) . I n addition to features
ol gross m o r p h o l o g y , the g l a n d u l a r s t r u c t u r e s reveal
g r e a t e r specialization than do those of other tarweeds i C a r l q u i s t . 195<>t with the possible exception ..|' (tilycadenia
(Carlquist. 1 9 5 9 ) . T h e g l a n d u lar s t r u c t u r e s of Holocarpha
can be divided into
1 g r o u p - : ( 1 ) biseriate t r i c h o m e s which o c c u r on
h e r b a g e and involucral b r a c t s ( C a r l q u i s t , 1 9 5 8 ;
see h e r e i n , fig. 6, 19, 3 0 ) ; in some of these, the
h e a d s mav be subdivided into m o r e n u m e r o u s cells;
(21 m u l t i s e r i a t e . short t r i c h o m e s o c c u r r i n g on
coroQa-lobe tips (Carlquist, 1 9 5 8 1 : ( 3 ) sessile
g l a n d s present at the tips of upper leaves a n d in-
trolucral and receptacular bracts: (4) hollowstalked, multiseriate trichomes on the outer faces
of involueral and receptacular b r a c t s . T h e latter 2
c a t e g o r i e s form the basis for t h e present paper.
1
Received for publication Julv 18. 1958.
T h e s t r u c t u r e of these types is u n i q u e a m o n g tarweeds. and ontogenetic studies were u n d e r t a k e n i<>
aid in clarifying the relation between these glandular s t r u c t u r e s and their simpler c o u n t e r p a r t s in
other g e n e r a of M a d i n a e . T h e sessile glands of
Holocarpha
a r e intimately related to the leaves a n d
involucral b r a c t s on which the) a r e b o r n e . O n t o genetic studies of the leaf itself a r e presented h e r e
on this account, together with d a t a on m a t u r e leaf
s t r u c t u r e . T h e relatively a d v a n c e d a n a t o m y of the
leaf ( c o m p a r e d with other M a d i n a e i provides pertinent c o m p a r i s o n s in assessing evolution of a n a t o m i cal c h a r a c t e r s within this s u b t r i b e .
MATERIALS AND M E T H O D S . — L i v i n g material of
Holocarpha heermannii
and of H. virgata in various
stages of development was collected in the field a n d
p o r t i o n s were preserved in f o r m a l i n - p r o p i o n o alcohol. Samples of H. macradenia
( w h i c h is p r o b ald\ now e x t i n c t : Clausen. 1951) and H. obconica
were taken from h e r b a r i u m s p e c i m e n s and e x p a n d ed in 2.5 r « a q u e o u s N a O H . Both types of material
were e m b e d d e d in paraffin a c c o r d i n g lo the usual
techniques. T h e s a f r a n i n - f a s t g r e e n staining series
used for sections c o r r e s p o n d s to N o r t h e n ' s modification of F o s t e r ' s t a n n i c a c i d - f e r r i c c h l o r i d e method
( J o h a n s e n . 1 9 4 0 1 . T h e pectic Substances alluded
April, 1959]
CARLQUIST—GLANDULAR STRUCTURES OF HOLOCARPHA
301
Fig. 1-7. Ontogeny of sessile gland in Holocarpha virgata.—Fig. 2-3. Primordia of involucral bracts; the remainder,
leaf primordia.—Fig. 6. Paradermal section; the remainder, sagittal sections of primordia.—Fig. 1. Leaf primordium just
prior to differentiation of initials. X350.—Fig. 2. Initials at tip of primordium. X475.—Fig. 3. Anticlinal divisions in
initials. X420.—Fig. 4. First periciinal division. X480-—Fig. 5. Additional divisions producing a flattened configuration
at tip of primordium. X580.—Fig. 6. Widening of future gland. X550.—Fig. 7. Stage at which cell division is nearly
complete. X370. Diagrammatic representations given in fig. 8-13.
302
[Vol. 46
AMERICAN JOURNAL OF BOTANY
to below were identified by means of their deep
coloration with Ruthenium Red and their solubility
in warm 0.5' , ammonium oxalate. Such pectic
compounds, however, also exhibited a tendency to
dissolve somewhat in the formalin solution used in
mounting the sections on slides. Pectic accumulations appeared to be retained in NaOH-treated
material to an appreciable extent.
Specimens which document preserved material,
or dried specimens from which material was taken,
are as follows: H. heermannii.
Carlquist 416
( R S A ) ; H. rnacradenia, McMurphv X-10-1909
(RSA) ; H. macradeniu. Jones VI-23-1887 ( P O M ) ,
basal leaves onl\ : H. obconica, Keck and Stockwell
2501 i RSA. isotype); //. virgata, Carlquist 412
I RSA). In addition, all the herbarium specimens
of Holocarpha in the Rancho Santa Ana Botanic
Garden (RSA) and Pomona College (POM) herbaria were examined for data on trichome distribution. The writer wishes to thank the curators of
the herbaria indicated above for the use of their
materials. Appreciation is expressed to Dr. David
D. Keck for bis interest in these studies.
SESSILE GLANDS.—The tips of upper leaves (chiefly those on abbreviated branches which terminate in
heads) and of involucral and receptacular bracts
bear a glandular disk (fig. 14, 15). Because the
nia!lire structure of these differs so sharply from
that of a trichome. the less precise term "sessile
gland"* is used here. The ontogeny of these is shown
in the photographs, fig. 1-7: most of these are
interpreted by line drawings in fig. 8-13. The first
perceptible stage in the development of sessile
glands is the enlargement of a group of protodermal
cells at the tip of a primordium I fig. 2. 9 ) . Prior
to this enlargement, all the protodermal cells are
alike in size and staining properties (fig. 1, 8 ) .
8
10
There is no visible evidence that only a single cell
is involved, although all the initials could conceivably have been derived ultimately from a single
protodermal cell earlier in the development of the
primordium. Differentiation of initials takes place
on very young primordia. Anticlinal divisions in
those cells destined to give rise to the gland (fig.
3, 10) widen the area of initials. This stage is followed by the appearance of periclinal divisions I fig,
4, 11) which increase the number of cell layers.
Because the walls which separate protoderm from
ground meristem stain more deeply, the derivation
of the gland from protoderm exclusively can be
demonstrated in young primordia. Cell enlargement
and both anticlinal and periclinal divisions occur in
such a manner that a blunt shape is imparted to
the tip of the primordium (fig. 5, 1 2 ) . Further
periclinal divisions (fig. 6) increase the number of
cell layers. Approximately three or four layers
(fig. 7, 13) are set up in this manner. Further
stages in the development of the gland are marked
by anticlinal divisions in these layers. The future
gland is widened I fig. 7, 131, and is in fact invariably wider at this stage than the primordium
which bears it. At this point (fig. 7) procambium
is present in the leaf primordium. Such a stage is
seen in transection in fig. 17. Relatively little
change in cellular constitution can be noted between
a gland such as that shown in fig. 7 and the mature
structure in fig. 14, 15. Cells of the gland now
become filled with a resin-like substance and commence their secretory activities. Meanwhile, the
vascular tissues of the leaf attain their characteristic
mature configurations. Although vascular tissue
of the midvein and its branches terminates near the
base of the gland ' fig. 14. 15 I. such vascular tissue
does not enter it. Prominent cell elongation and
12
13
Fig. 8-13. Diagrammatic drawings of the sagittal sections of primordia illustrated in fig. 1-5, 7.—Fig. 8 = fig. 1.
X350.—Fig. 9 - fig. 2. X330.—Fig. 10 = fig. 3. X330.—Fig. 11 = fig. 4. X370.—Fig. 12 = fig. 5. X380.—Fig. 13 =
fig. 7. X225. Heavy line separates the protoderm and its derivatives from ground meristem. Broken lines represent probable recent divisions.
Fig. 14-19. Holocarpha virgata.—Fig. 14-15. Longitudinal sections of tip of upper leaf, showing sessile gland.—Fig.
14. Paradermal section. X240.—Fig. 15. Sagittal section (adaxial side below). X270.—Fig. 16-18. Transections of primordia of upper leaves, taken approximately midway along primordia which are .09, .14, and .72 mm. long, respectively. X350.—Fig. 19. Transection of upper leaf, taken about midway along length of the leaf. X195.
April, 1 9 5 9 ]
CARLQIIIST—GLANDULAR STRUCTURES OF HOLOCARPHA
303
304
AMKRICAN JOURNAL OF BOTANY
enlargement occur in palisade and spongy tissues
of the mesophyll, so that the leaf becomes wider
than the gland.
Because several protodermal initials are involved
in the formation of the gland, rather than a single
initial, the gland is not trichome-like in its ontogeny.
Even the massive tack-shaped glands on upper leaves
of Calycadenia originate from a single cell (Carlquist, 1959). In these tack-shaped glands, however, the single cell often divides into four cells
which, in their subsequent division products, are
not trichome-like, but reminiscent of those of sessile
glands of Holocarpha.
The fact that only protodermal initials are involved in the formation of the
sessile glands of Holocarpha does suggest that these
glands represent derivatives of trichornes. phylogenetically. Because all other glandular structures
in Madinae are apparently of trichome origin, the
sessile glands of Holocarpha could hardly have been
derived from any other structure. Their restriction
to the tips (or teeth I of leaves is like the restriction
of tack-shaped glands to tips of leaves in some
species of Calycadenia (Carlquist. 1959). If the
sessile glands of Holocarp/ui are oi trichome origin,
however, their ontogeny has been highly altered.
One can formulate a series within Madinae of progressively more advanced structures. In such a
series, the biseriate stage is pushed further back
into ontogeny.
Biseriate trichornes retain this
character throughout their development. Capitate
trichornes with biseriate stalks show the biseriate
condition until subdivisions in the terminal portion
alter this. The tack-shaped trichornes of Hlepharizonia (Carlquist. 1958) are biseriate only until
four tiers of cells are formed, at which time a series
of anticlinal divisions initiates a quadriseriate condition. A similar situation obtains in the smaller
tack-shaped glands of Calycadenia
i Carlquist,
1959). In the larger terminal glands of that genus,
however, the biseriate staae consists only of a pair
of cells which subdivide directly to a quadriseriate
condition. Therefore, the lack of a biseriate stage
in the highly advanced sessile glands of Holocarpha
is not surprising. Concomitant with the progressively greater alterations in trichome-like ontogeny.
there is a tendency for glandular trichornes, or
glands, to be initiated earlier in ontogeny (i.e., on
progressively smaller primordial.
LEAF ANATOMY.—As in other genera of tarweeds,
leaves of Holocarpha are polymorphic within a single plant. The upper leaves seem to represent modifications, in certain respects, of the structure found
in lower leaves. The peculiarities in structure of
upper leaves are illustrated in fig. 16-19. As fig.
16-17 show, a central strand of procambium becomes evident at an early stage. This strand of
procambium is destined to become the midvein.
Procambium for all other veins originates as
branches of this single strand. A stage in such
[Vol. 46
branching is shown in fig. 18. Differentiation of
the ground meristem of the primordium into a
hypodermal layer, which subdivides into the 2
layers of palisade which surround the leaf, and the
future spongy parenchyma cells, also is shown in
fig. 18. The mature leaf (fig. 19: see also. fig.
I 1- 15 I is cylindrical hi construction. The midvein
(visible also in longitudinal section, fig. 15) lies
beneath a layer of palisade parenchyma on the
adaxial surface. The other veins form a network
which encircles the leaf, so that a cylindrical con-tiuction is evident. In the basal portion of the
leaf (fig. 2 0 1 , the bundles lie in a single plane. At
the leaf base, all the lateral bundles join the midvein, and a unilacunar node is present. The transition between the single plane of bundles at the
base of the leaf and the cylinder of veins above the
base involves a curving of lateral bundles around
toward the lower surface of the leaf, so that they
meet along most of the length of the leaf and form
a cylindrical conformation. Additional features of
histologv in upper leaves of Holocarpha merit mention. The outer walls of epidermal cells are extremel] thick-walled. Apparently no cuticle is present on mature leaves. No demarcation of cuticle
from cell wall can be observed, and the lack of
staining by safranin suggests absence of cutin. Internal walls of the epidermis are also rather thick.
Cells of the bundle sheaths are often lightly scarified, and a few fibers may be present near the
phloem pole of the midvein. No bundle-sheath
extensions are present. Of considerable interest is
the spongv mesophyll in the center of the leaf. In
the intercellular spaces between these cells, pectic
compounds are accumulated, partly or entirely fillin- these spaces. Thus the center of the leaf is
converted into a pectic channel, much like those
described by the writer for other members of
Madinae. Argyroxiphium
(Carlquist. 1957) and
Calycadenia (Carlquist. 1959).
Leaves of the main stem of Holocarpha virgata
(fig. 21 l show less specialization in vascular structure. They do show an incurving of bundles at the
margins. This fact is reflected in the gross morphology of the leaf, for the "true" margin is recurved toward the lower surface. This incurving
of the margins and marginal bundles is not complete, so that a cylindrical structure is not attained.
The midvein is jacketed by a sclerified bundle
sheath which is contiguous to fibers at the phloem
pole of the bundle. A sheath extension, consisting
of thin-walled parenchyma, is present. Because the
sheath extension separates the two areas of spongy
parenchyma and their intercellular pectic contents,
two pectic channels may be said to be present.
Some leaves of the main stem may be thinner than
the one figured. In this case, some lateral bundles
are in contact with both upper and lower palisade.
Thus, more than 2 pectic channels may be present.
April. 1959]
CARLQUIST—GLANDULAR STRUCTURES OF HOLOCARPHA
Leaves of the basal rosette of H. virgata show a
"normal" vascular structure in that the bundles lie
in a single plane and no incurving of the margin is
present. This condition is shown here for H.
macradenia (fig. 2 2 ) . Such leaves are isolateral
in respect to palisade parenchyma distribution. Unlike upper leaves, they are associated with trilacunar
nodes. A prominent marginal strand of fibers is
present interior to the marginal vein; this feature
is characteristic of many other tarweeds, such as
Calycadenia (Carlquist, 1959). Bundle-sheath extensions and fibers adjacent to phloem may be present on some of the lateral veins of basal leaves. In
gross morphology, leaves of the basal rosette differ
from upper leaves in that teeth are present along
the margins of basal leaves.
The variation of structure from leaves of the
basal rosette to those of the main stem and those
on upper, lateral branches may be found in all
species of Holocarpha.
In H. macradenia. however, this transition is incomplete, because a cylindrical construction is not attained by upper leaves.
Upper leaves of H. macradenia correspond to the
main stem leaves of H. virgata illustrated in fig. 21.
Thus the leaf morphology of H. macradenia illustrates a retention of a more juvenile form, or a
less evolved condition, than the other three species.
All species of Holocarpha agree in possessing
persistent capitate liichomes with biseriate stalks.
These occur on basal and upper leaves (fig. 19).
On basal leaves, these trichomes tend to have longer
stalks, resembling those figured for Madia saliva
(Carlquist, 1958). The teeth of a few basal leaves
may be converted into sessile glands. This was
observed in some specimens of H. macradenia, H.
obconica. and H. virgata. Leaves possessing such
glandular teeth are not present on every plant of
these species, however. The species of Holocarpha
all possess uniseriate non-glandular trichomes in
addition to the glandular ones. These trichomes
may be long and several cells in length, and were
observed on all leaves and on the stem of all specimens examined of H. macradenia and on some of
H. heermannii.
There appears to be a tendency
toward glabrescence. however, because upper leaves
of some specimens of H. heermannii and nearly
all specimens of H. obconica and H. virgata show
very short uniseriate non-glandular trichomes or
none at all. No secretory canals were observed in
any leaves of the species of Holocarpha.
HOLLOW-STALKED TRICHOMES OF T H E INVOLUCRE.
—In addition to sessile glands, the species of Holocarpha possess a type of glandular trichome which
does not occur in any other tarweed. These are
borne on the outer surface of involucral and receptacular bracts. They are distinctive in possessing a
massive glandular head and a hollow cylindrical
stalk, the walls of which are a single cell in thickness (fig. 23, 24, 3 1 ) . The ontogeny of these tri-
305
eliomes illustrates that, unlike sessile glands, the
development suggests that of other trichomes of
Madinae. Hollow-stalked trichomes originate from
a single cell (fig. 25. right). An anticlinal division
in a plane parallel to the long axis of the primordium initiates a biseriate condition (fig. 26; fig. 3 2 ) .
Periclinal divisions then produce several tier- <>\
cells in this biseriate structure, so that at this stage
it corresponds to the simple biseriate trichomes of
other Madinae. Anticlinal divisions then convert
the biseriate structure to a quadriseriate condition.
These divisions begin at the base of the trichome
(fig. 25, left; fig. 33. 34) and then extend to the
terminal tiers (fig. 27, right; fig. 3 7 ) . At this
stage, the trichome is not unlike the quadriseriate
stage in the development of tack-shaped trichomes
of Blepharizonia
(Carlquist, 1958). Subsequent
steps in development of the hollow-stalked trichomes
are unique, however. In the future stalk portion,
anticlinal divisions form radially to the center of
the four series of cells. Because of this radial
orientation of all divisions in the future stalk, followed by formation of a space in its center, the
wall of the stalk is only a single layer of cells in
thickness (fig. 3 5 ; fig. 36, mature). This space
results from separation of cells along their inner
faces. Divisions in the future head of the trichome
are not radial to the center of the trichome I tig.
38), but blocks of cells are set off so that by subdivision in various planes a massive multicellular
structure is initiated I fig. 39). No intercellular
space develops among these cells. A young trichome in which the stalk has begun to develop its
hollow center is shown in fig. 28. Divisions in the
head portion of this trichome are not yet complete.
Such divisions begin in the subterminal lasers.
and the terminal layer is subdivided last. Three
cell tiers are participating in the formation of the
head of the trichome seen in fig. 28. A longitudinal
section of a trichome. cut at right angles to that
shown in fig. 28, is illustrated in fig. 29. This trichome represents a later stage in which the hollow
nature of the stalk is clearly visible. The terminal
portion of the trichome has been widened by
means of numerous anticlinal divisions. The trichome in fig. 30 (cut in the same plane as that of
ficr. 28) illustrates approximately this same stage.
Further development of the trichome is characterized by elongation of cells of the stalk. A
significant feature which accompanies this, however, is the intrusion of ground meristem cells
of the bract into the hollow stalk. The beginning
of such intrusion is visible in fig. 30. The extent
of this intrusion in mature hollow-stalked trichomes
of H. virgata may be seen in fig. 31. Of particular
interest is the fact that in such intrusive ground
meristem of many trichomes, procambium is formed. Maturation of intrusive tissues results in the
formation of a vascular bundle surrounded by palisade parenchyma. In the trichome shown in fig.
306
VMKRICA.N .IOI HNAL OF BOTANY
[Vol. 46
Fig. 20-24.—Fig. 20. Holocarpha virgata. Transection of basal portion of upper leaf. X135.—Fig. 21. H. virgata. Transection of leaf of main stem, cut midway along length of leaf. X130.—Fig. 22. H. macradenia. Transection of leaf of
basal rosette, taken midway along length of leaf. X180.— Fig. 23. H. macradenia. Hollow-stalked trichome from sagittal section of involurral bract. X180.—Fig. 24. H. heermannii. Hollow-stalked trichome from sagiltal section of inwolucral bract. X215.
April, 1959]
CARLQUIST—GLANDULAR STRUCTURES OF HOLOCARPHA
307
Fig. 25-39. Stages in development of hollow-stalked trichomes on involucral bracts of Holocarpha virgata.—Fig. 2526. From oblique longitudinal sections.—Fig. 27-28, 30-31. From sagittal sections. Fig. 29 from transection.—Fig.
32-39. From paradermal sections above the surface of bract.—Fig. 25. One-cell stage. X715.—Fig. 26. Two-cell stage.
X790.—Fig. 27. Quadriseriate stages. X715.—Fig. 28. Opening of stalk. X600.—Fig. 29. Widening of head region.
X610.—Fig. 30. Immature trichome, showing elongation of stalk and intrusion of mesophyll into stalk base. X285.—Fig.
31. Mature trichome. X150.—Fig. 32-36. Stages in development of stalk, a- « m in transection of trichome.—Fig.
37-39. Stages in development of head, as seen in transection of trichome.—Fig. 32-35, X485; fig. 36, X200; fig. 37,
39, X430; fig. 38, X550.
:;<>:;
[Vol. 46
AMERICAN JOURNAL OF BOTANY
31, the vascular bundle (not clearly distinct from
palisade cells) runs nearly to the end of the intruded
tissue. The intrusive mesophyll and its vascularization may be clearly seen in the trichome of H.
macradenia shown in fig. 23. This vascular tissue
represents a branch from one of the veins of the
involucral bract.
In the mature trichome, secretory activity is
indicated by the resin-like contents of the cells
of the head portion. In the fresh condition, a droplet of this resin-like substance covers the head,
just as a droplet covers the disk of sessile glands.
Because of the massive group of secreting cells,
there is a certain similarity in anatomical appearance between sessile glands and the terminal
portion of hollow-stalked trichomes. One might
liken the formation of a sessile gland to the development of a hollow-stalked trichome without
the stalk. The absence of secretory canals in
leaves and bracts of Holocarpha, as opposed to
those of other tarweeds. in which canals are present, might be correlated with the variety and
abundance of surface structures which apparently
secrete the same type of resin-like substances as
do the canals.
Although all the species of Holocarpha are alike
in respect to their sessile glands, there are differences among the species in respect to the hollow-stalked trichomes. The type illustrated for
H. virgata also occurs in H. obconica. In H. heermannii, however, the hollow-stalked trichomes are
smaller and more numerous on each bract. The
smaller size is evident in the section shown in fig.
24. The stalk portion of the trichome is shorter
than in H. virgata. Also, there is less intrusion of
mesophyll into the stalk, and vascular tissue is
less frequently present. As the specific name MIL;
gests, the largest hollow-stalked trichomes in tli«genus occur in H. macradenia (fig. 23). In cellular
constitution they are not markedly different from
those of H. virgata. but they are longer and wider.
Intrusive mesophyll in the base of the trichome is
generally more abundant, and the presence of a
vascular bundle within the intrusive mesophyll is
usually more frequent and more conspicuous than
in H. virgata.
DISCUSSION.—This and the preceding papers in
this series (Carlquist, 1958, 1959) have attempted
to show that the glandular structures of the tarweeds are exceptionally varied in form, size, and
structure. Just as patterns of evolution in the
tarweeds provide interesting problems which have
been studied from genetic, taxonomic. and physio-
logic standpoints by Clausen, Keck, and Hiesey
(e.g., Clausen, 1951), the glandular structures of
Madinae offer exceptional material for the study
of evolution in anatomical characteristics.
Investigation of ontogeny of these glandular structures is requisite for such a study, because alterations in the mature structure are best traced from
their point of origin in ontogeny. The simple
biseriate glandular trichome, which probably occurs in the majority of genera of Compositae, is
represented throughout Madinae. From this trichome. the capitate trichome has evolved bv means
of additional divisions relatively late in the ontogeny of the terminal portion of the trichome.
From such capitate trichomes. the more specialized
tack-shaped trichomes of Hemizonia fttchii, Blepharizonia. and Holozonia have been developed. The
most complicated products of such evolution are
found in the genus Calycadenia. which bears peculiar tack-shaped glands in the stalks of which
vascular tissue is often present. Equallv advanced
in their own wavs are the two tvpes of glandular
structures of Holocarpha described above. Increasomplexity of glandular structures is accompanied by progressively earlier initiation (i.e.,
initials of more complicated types appear on smaller primordia). In Calycadenia and Holocarpha,
the ontogeny of these glandular structures is intimately related to that of the leaf or bract itself,
and tissue svstems other than the epidermal become involved in mature glandular structures.
Complexity of glandular structures is parallelled
bv the specializations in the structure of the leaves
which bear them.
CLAREMONT GRADUATE SCHOOL
RANCHO SANTA ANA BOTANIC GARDEN
CLAREMONT. CALIFORNIA
LITERATURE CITED
Cuu.onisT, S. 1957. Leaf anatomy and ontogeny in Argyroxiphium and Wilkesia (Compositae). Amer. Jour.
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