O. T. (Orville Thomas) Bonnett.

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no. 672
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* T



Its Histology and Development



Its Histology and Development



Urbana, Illinois June, 1961

Publications in the bulletin series report the results of investigations

made or sponsored by the Experiment Station



Literature Review 5

Methods and Materials 6

General Morphology 6

Stages of Plant Development 7


Shoot Apex 8

Epidermis: General Characteristics 10

Foliage Leaf 16

Epidermal tissue 17

Mesophyll tissue 19

Vascular tissue 21

Mechanical tissue 24

Collar 25

Ligule 25

Leaf sheath base 25

Stem 27

Node 27

Internode 29

Epidermal tissue 31

Mechanical tissue 31

Ground tissue 32

Vascular tissue 32

Tiller 33

Roots 35

Rootcap 38

Origin of epidermis, cortex, and stele 38

Epidermal tissue 39

Cortex 41

Stele 42


Mature Panicle 43

Development of the Panicle 45

Spikelet 50

Rachilla 51

Floral axis 53

Spikelet differentiation 54

J. /


Spikelet continued PAGE

Empty glumes ...................................... 54

Lemma ........................................... 57

Awn ............................................. 60

Palea ............................................ 62

Lodicules .......................................... 63

Stamens .......................................... 66

Pistil ............................................. 73

Styles ......................................... 75

Stigma ........................................ 77

Ovary ......................................... 79

Ovule ......................................... 83

Embryo sac ..................................... 84

Integuments .................................... 86

Trichomes ...................................... 87


Mature Grain ......................................... 87

Pollination ............................................ 88

Fertilization ........................................... 90

Embryogeny .......................................... 93

Scutellum ......................................... 96

Epiblast ........................................... 98

Coleoptile ......................................... 99

Foliage leaves ..................................... 101

First internode ..................................... 1 02

Roots ........................................... 1 05

Endosperm ....................................... 108


A list of the illustrations is on the following page.


Photomicrographs of sections of the shoot apex PAGE

and of foliage leaves (Fig. 1 ) 9

Epidermis (Fig. 2) 1 2

Development of stomata (Fig. 3) 1 4

Mesophyll of the foliage leaf and a trichome (Fig. 4) 20

Leaf sheath (Fig. 5) 22

Stages in the development of the ligule (Fig. 6) 26

Stem (Fig. 7) 28

Tillers (Fig. 8} 34

Embryonic and young roots (Fig. 9) 37

Young and mature roots (Fig. 1 0) 40

Mature oat panicle and the nodes of its main axis (Fig. 11) 43

Stages in the development of the oat panicle (Fig. 12) 44

Longisections of early stages of panicle initiation (Fig. 13) 46

The pulvinus, the internodes of the main axis,

and the branches of the panicle (Fig. 14) 49

Spikelets and spikelet parts (Fig. 15) 52

Glumes, lemmas, and paleas (Fig. 16) 56

Initiation and development of the lodicule (Fig. 17) 64

Development of the stamen and early stages of the anther. . .(Fig. 18) 67

Anthers (Fig. 19) 69

Anther and filament (Fig. 20) 70

Development of the pistil (Fig. 21 ) 74

Early stages in the development of the pistil (Fig. 22) 76

Style (Fig. 23) 78

Stigmatic branches of the style (Fig. 24) 80

The embryo sac and megasporogenesis (Fig. 25) 82

Egg, polar nuclei, and antipodal cells (Fig. 26) 85

Pollen and pollen tube growth (Fig. 27) 89

Fertilization of the polar nuclei and egg (Fig. 28) 92

Early stages in the development of the proembryo

and of the embryo (Fig. 29) 94

Embryo development (Fig. 30) 97

Transections of a mature embryo at different levels (Fig. 31) 99

First internode of the oat seedling (Fig. 32) 1 03

Radicle (Fig. 33) 106

Development of the endosperm (Fig. 34) 1 09

Its Histology and Development

By O. T. BONNETT, Professor of Plant Morphology

initiation, development, and histological characteristics of each of
the plant parts and the pollination, fertilization, and embryogeny of
the plant. Photomicrographs supplement the description.

The treatment of the subject matter is complete but not exhaustive.
No attempt has been made to recite all of the variations in the struc-
ture and development of the various plant parts. The number of such
details is almost unlimited, and a full understanding of the variations
in structure and development can only be gained first-hand. However,
those plant parts that are sometimes overlooked or incompletely de-
scribed have been given attention. The purpose has been to supply a
source of information for the plant breeder, plant pathologist, or
agronomist who may be in need of a better understanding of the
structure and development of the oat plant.

Literature Review

Though the literature on the morphology of grasses is extensive,
the number of publications on the oat plant is small. In the publication
reporting her extensive studies of cereal, bamboo, and grass, Arber
(1934) gave some attention to the oat. Esau (1953) included certain
characteristics of the oat among her descriptions of the Gramineae.
Stanton (1955) gave excellent detailed descriptions and illustrations
of the parts of the oat plant that are useful in the identification of
Avena. The shoot apex of the oat plant was described by Kliem
(1936), Hamilton (1948), and Holt (1955). Among other topics,
panicle development was included by Bonnett (1937) and Holt (1955).
Development of the spikelet, floret, fertilization, and early embryogeny
of Avena fatua L. were accurately described by Cannon (1900). Re-
cently Brown and Shands (1957) studied pollen tube growth in the
pistil and gave the time intervals for the events of fertilization and the
first divisions of the embryo.

Many of the details of development and histology described for
other grasses are found to be similar in the oat. Some of these descrip-
tions will be referred to in the text.

6 BULLETIN No. 672 [June,

Methods and Materials

Several varieties of cultivated spring oats were used in this study.
Clinton C.I. No. 4606, Columbia C.I. No. 2828, and an early maturing
Canadian selection, Valor, provided most of the material.

Standard histological techniques were used. Formalin-acetic acid-
alcohol and Craf (Randolph, 1935) and Craf II and III, both as
modified by Sass (1951), were used for killing and fixing material for
paraffin sections and for clearing. The tertiary butyl alcohol method
described by Johansen (1940) was used for dehydrating and em-
bedding material to be sectioned with a microtome. Most of the stain-
ing was done by the tannic acid-iron alum, Safranin O and Orange G
procedure of Sharman (1943). Other staining procedures were used
to study special details, such as an iodine-potassium iodide solution to
stain pollen grains and pollen tubes in the styles (Pope, 1946).

Cleared material was used for determining the course of vascular
bundles and for studying the position and relation of the parts to each
other. Clearing was done with a mixture of equal parts by volume of
chloral hydrate and of phenol crystals which, when mixed, form a
liquid. The plant material was placed in the liquid and heated until
the part was clear. After clearing, the plant material was placed in
lactic acid for storage and subsequent observation. Cleared material
was also washed in several changes of 50-percent ethanol until the
clearing solution was removed. The cleared material was then dehy-
drated in an ethanol series to xylene and mounted.

General Morphology

The cultivated oat is an annual grass. Its major morphological
characteristics are typical of grasses in general. The oat stem, or culm,
is composed of a series of nodes and internodes, with alternate solitary,
sessile leaves in two ranks. The mature stem may terminate in a loose,
open panicle. The panicle consists of a main axis terminating in a
single, multiflorous spikelet. Alternate groups of branches are found
along the main axis, each branch terminating in a single, multiflorous
spikelet (Fig. 11, A). The fruit is a caryopsis furrowed on the side
opposite to the embryo. The caryopsis consists of a starchy endosperm
and of an embryo inclosed together within the walls of the caryopsis.
In addition to leaves, stems produce intervaginal, axillary branches
(tillers). The fibrous root system consists of two groups: the seedling
roots and the permanent roots.


Stages of Plant Development

The life cycle of the oat plant from germination to the mature
seed can be divided into four stages. They are the vegetative, transi-
tion, reproductive, and seed stages (Bonnett, 1937). Each of these
stages can be distinguished from the others by its morphological char-
acteristics and by the developmental events that occur in it. In the
vegetative stage, leaves, axillary shoots (tillers), and the permanent
root system (adventitious roots) are initiated and develop. The apex
of the main shoot (Fig. 12, A, B) and the apexes of the tillers remain
short and round at the tip during this stage, and the stem internodes
are usually very short. The transition stage is of short duration and
more difficult than the other stages to identify in the oat since it con-
sists only of a slight elongation of the shoot apex preceding the initia-
tion of the panicle. During the reproductive stage, the panicle and its
parts differentiate and develop, and the internodes of the stem elon-
gate. The seed stage begins with fertilization and terminates with the
maturation of the seed. Each stage is terminal in the sense that the
kinds of parts initiated in a given stage are not initiated in a later
stage. For example, the initiation of foliage leaves, of nodes, and of
tiller primordia terminates with the transition stage, which, in its turn,
is followed by panicle initiation in the reproductive stage. However,
while the initiation of the tiller primordia terminates in the vegetative
stage, tillers may grow and develop during the formation of the panicle,
and they sometimes appear as "second growth" after the seed has
matured on the plant.

The various developmental stages in spring-seeded oats are not
equally long. The length of the vegetative stage varies with the time
of seeding because in a given location and with a given variety or
maturity group, panicle initiation will begin at approximately the same
date each year. However, the length of time of the reproductive stage
is nearly constant from year to year for any given variety or maturity
group. The duration of the seed stage is likewise nearly constant. The
two stages are not, however, of identical duration (Ross, 1955).


At germination the developmental events that were initiated in the
embryo are resumed. The events initiated in the embryo will be
described in detail in the section on embryogeny. When the caryopsis
is mature, the shoot (plumule) of the embryo consists of two leaf

8 BULLETIN No. 672 [June,

primordia and the shoot apex. The shoot apex is inclosed by the first
leaf, and the entire shoot is inclosed by the coleoptile. The root sys-
tem consists only of the seminal roots: the primary root, or radicle,
and two, three, or more adventitious roots. The radicle is covered by
the coleorhiza. When growth is resumed, at germination, the coleoptile
and the leaves of the shoot elongate, and soon the leaves of the shoot
break through the coleoptile. At the same time, the first internode of
the stem elongates and raises the shoot, if planting is at a depth of 1 or
more inches, to a point just beneath the surface of the soil. Leaves are
initiated beneath the apex of the shoot, and axillary shoots (tillers) and
the adventitious roots of the permanent root system are initiated in the
stem internodes.

Shoot Apex

The apexes of the main and lateral shoots, together with the apexes
of the roots, constitute the apical meristems of the oat plant. The
apical meristems are self -perpetuating. The lateral organs and the
structures of the stem and root are produced from their cell derivatives
(Figs. 1, A; 9, A).

During the vegetative stage, the shoot apex is a short hemisphere.
The length and shape of the shoot apex changes from the beginning
of the plastochron, when the apex is shortest, to the end of the plasto-
chron, when it is longest. The shoot apex also increases in length and
diameter as the plant approaches the reproductive stage.

The cytohistological zones present in the shoot apex of the oat are
characteristic of the usual angiosperm type described by Popham
(1951). They consist of the mantle (tunica), the subapical initials, a
peripheral zone, and a central meristem (Fig. 1, A). The mantle

Photomicrographs of sections of the shoot apex and of foliage leaves.

A. Apex of the main shoot in the vegetative stage. X230.

B. Longisection of foliage leaf primordia. X287.

C. Transection of parts of some foliage leaves at an early stage of develop-
ment. X66.

D. Transection of a portion of a foliage leaf, showing the bulliform cells in
the upper surface of the leaf. X66.

E. Transection of a foliage leaf at the margin. X66.

F. Transection through the midrib of a leaf. X 100.

G. Transection of a medium-sized vascular bundle of a leaf. X230.

(bl = bulliform cells; cm = central meristem; f = fibers; in = initiation of
leaf; Ip = leaf primordium; m = mantle; me = mesophyll; mt = mestom;
pe = peripheral meristem; sa =subapical initials; vb = vascular bundle;
x = periclinal cell divisions)



(Fig. 1. Sec opposite page for legend)

10 BULLETIN No. 672 [June,

consists of a single cell layer (Kliem, 1936) which develops eventually
into the epidermal layer. Initiation of foliage leaves and leaf-like
parts begins in the mantle. The subapical initials are composed of a
self -perpetuating group of a few irregularly shaped cells that may
divide in any plane. The derivatives of the subapical initials form
the peripheral and central meristem zones. Procambial strands and
lateral shoots are initiated in the peripheral zone. Ground, or funda-
mental tissue (usually called pith), develops from the cells of the
central meristem.

Epidermis: General Characteristics

The general characteristics common to the oat epidermis on all
parts of the oat plant will be described here preceding the description
of the specific characteristics of the epidermis of the leaf. Specific
characteristics of the epidermis of other parts or structures will be
given later.

The oat, Avena, belongs to the Festucoideae, a group of grasses
whose epidermal cells are rather simple in shape: their hairs are
unicellular, and their silica cells are mostly round or elongate (Prat,
1936, 1948). The epidermis originates from the protodermal (outer-
most) layer of cells of the proembryo and later from the protodermal
layers of the apical meristems. Habertandt's term, protoderm, was
used by Esau (1953) for the undifferentiated epidermis. Prat (1948)
classified the epidermal cells into three groups: (a) fundamental cells,
consisting of long cells with straight, or undulating, or deeply fur-
rowed walls; (b) differentiated cells consisting of silica, cork, tri-
chomes, guard, and subsidiary cells; and (c) bulliform cells.

Cells of the protoderm are square or short rectangles, with thin
walls, large nuclei, and dense, darkly staining cytoplasm. Cell divisions
in the protoderm are mostly anticlinal or, if viewed from the surface,
at right angles to the long axis of the structure covered. At first the
primordium of the leaf, shoot, or root is covered by the protoderm,
but later the protoderm is restricted to the intercalary zones of meri-
stematic tissue, and finally it disappears as the plant matures. Differ-
entation of the epidermis is basipetal. For example, differentiation of
the epidermis is from the tip of the leaf toward the base or, on the
stem, from the node toward the base of the internode. The leaf and
the stem mature in the same order.

Epidermal cells are arranged end to end in rows that extend the
length of the structure covered. The cells of a row may all be of one
kind, or a row may consist of combinations of cell types. More than


one row of any cell type or combination of cell types may be found
side by side, forming a strip that may extend for a considerable dis-
tance or for the entire length of a plant part. Fundamental cells,
bulliform cells, or cells with thick, lignified walls may occur in rows
consisting of only a single type of cell. Rows of combinations of cells
consist of fundamental cells with stomata, with silica and cork cells,
or with trichomes. The type of wall of the fundamental cells varies at
different places on the plant part or on different plant parts.

Fundamental cells constitute the greatest proportion of the epi-
dermis (Fig. 2, A, B, C, D). Differentiation of the fundamental cells
consists first of the elongation of the protodermal cells and, then, of
the development of their special characteristics. In those fundamental
cells that have undulating or deeply furrowed walls at maturity (Fig.
2, E), undulations appear while the cell walls are thin. As the cells
mature, the undulations become deeper and the walls thicker. The
most extreme type is the thick-walled, deeply-furrowed fundamental
cell of the lemma or palea.

Almost without exception, silica and cork cells are found in com-
bination with fundamental cells having undulating or deeply furrowed
walls (Fig. 2, D, E). This combination is found on the external sur-
face of the empty glumes, on the lemma and palea, on the basal half
of the leaf sheath, on the culm near the nodes, and on the main axis
and branches of the panicle. Silica and cork cells result from a series
of divisions within the row of cells in which they occur. A proto-
dermal cell divides at right angles to the long axis of the plant part.
The basal cell of the pair elongates, but the apical member remains
short, very little larger than the nucleus and with dense, darkly stain-
ing cytoplasm. The elongating cells usually alternate with the short
cells, but two or more long cells may occur between the short cells.
The nucleus of the short cell divides at right angles to the long axis of
the plant part to produce a pair of short cells which become the silica
and cork cells (Fig. 2, E, F, si and ck). The silica cell, which in the
oat is round, is always toward the apex of the plant part. The cork
cell, which is the basal member of the pair, is shaped approximately
like a half moon, with the open side of the crescent toward the silica
cell. The silica cells are filled with SiO 2 . The cork cells have suberized,
impermeable walls.

Silica is also deposited in the walls of other cells of the epidermis
as well as in the lumen and walls of silica cells (Bonner, 1950). Miller
(1938) stated that 46 percent of the ash of the oat plant is silica. If
the cellulose is oxidized with sulphuric and chromic acid, silica skele-
tons of the cells can be obtained (Zimmerman, 1901). Silica skeletons


v . . .* F 4

(Fig. 2. See opposite page for legend)


of all types of cells of the epidermis, as well as of fibers and of xylem
elements, have been obtained from oat stems, leaves, lemma, and palea.
Some silica skeletons of the epidermal cells are shown in Fig. 2, G.

Two pairs of cells, the guard cells and the subsidiary cells, are
associated with each stoma (Fig. 3, A, G, s and g). The guard cells
are on either side of the stoma, and a subsidiary cell is on the other
side of each guard cell. The guard and subsidiary cells are oriented
with their long axes parallel to the long axis of the plant part. Mature
guard cells have thick walls bordering the stoma, but at either end the
walls are thin. The ends of a guard cell are enlarged, so that seen from
the surface, it has the shape of a dumbbell. The thin-walled subsidiary
cells are the same length as the guard cells and lie adjacent to them.
In the oat, the guard and subsidiary cells form an ovoid-shaped group.

The guard cells and the subsidiary cells do not originate from the
protoderm (Fig. 3, B) in the same way (Campbell, 1881; Porterfield,
1937). Two divisions occur in the production of the guard cells. A
protodermal cell in the row in which the stoma are formed divides to
produce two cells. The basal member of the pair elongates and be-
comes the fundamental cell of the combination. The apical member of
the pair remains short and is the cell from which the guard cells are
produced (Fig. 3, C, gm and fd). Before it divides, the short cell
elongates slightly, becoming rectangular, and the nucleus divides
parallel to the long axis of the plant part. A wall forms around each
of the nuclei to produce each of the two guard cells (Fig. 3, E, g). As
the guard cells mature, an opening, called the stoma, is formed between
them (Fig. 3, A). Prior to the division of the nucleus of the short
cell, the nucleus of each adjacent fundamental cell comes to rest
opposite the short cell and divides parallel to its long axis. Each new
nucleus adjacent to the short cell is inclosed by a wall to form a
subsidiary cell that is the same length as the short cell (Fig. 3, D, E, s).
The two subsidiary cells may or may not develop simultaneously.
Rarely, one or both of the subsidiary cells may be lacking.

Epidermis. (Fig. 2)

A. Upper (adaxial) epidermal layer of a leaf blade. X66.

B. Lower (abaxial) epidermal layer of a leaf blade. X66.

C. Inner (adaxial) epidermal layer of a leaf sheath. X66.

D. Outer (abaxial) epidermal layer of a leaf sheath. X66.

E. Silica and cork cells. X100.

F. Longisection of a silica and a cork cell. X647.

G. Silica skeletons of epidermal cells. X287.

(bl = bulliform cell; ck = cork cell; si = silica cell)




Unicellular trichomes are epidermal appendages that are found on
all parts of the oat plant. They may be short, pointed, and hook
shaped or long with sharp points. The hook-shaped trichomes regu-
larly occur on the surface and margins of leaves and of leaf-like parts
(Fig. 4, E) and on the main axis and branches of the inflorescence.
They are usually over the vascular bundles with the point of the
trichome toward the apex of the plant part. Trichomes may also be

(Fig. 3. See opposite page for legend)


found between vascular bundles, with the point toward the base in
most cases. The long, pointed trichomes are found on the ovary and
along the keels of the palea. The apex of the lemma, the palea, and the
ligule and the margins of the lemma and the palea near the apex
terminate in sharp-pointed epidermal cells that are like trichomes, but
in the strictest sense of the term, they would not be called trichomes.

Initiation of a trichome occurs in the protoderm. A good place to
observe this process is at the margin of the blade of a young leaf.
Every other cell at the margin of the leaf forms a trichome. The first
indication of the initiation of a trichome by one of these cells is a pro-
trusion of its outer tangential wall. The protrusion continues to elon-
gate and turns toward the apex of the plant part, and the base of the
trichome elongates. A sharp tip is formed, and the cell becomes hook
shaped. The differentiation of a long, pointed trichome consists pri-
marily in the protrusion and marked elongation of the outer tangential
wall of the cell of the protoderm and in the development of a sharp tip.
The cytoplasm extends to the tip of the long, pointed trichomes, and
the nucleus is located about halfway between the base and the tip of
the cell. Thick walls extend to the tip of the trichome.

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