The shoot apex of Zea mays

120 Pages·4.397 MB·English

by  Ledin, Robert B

T H E S H O O T A P E X OF Z E A MAYS
R. BRUCE LED IE
Submitted to tne Faculty of the Graduate School in
partial fulfillment of the requirements for the
degree, Doctor of Philosophy, in the Department of
Botany, Indiana University•
ProQuest Number: 10296458
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TABLE OF CONTENTS
INTRODUCTION ................................................................... » 1
PREVIOUS WORK ON GRASS SHOOT APICES 4
MATERIALS AND METHODS ............................................................................. 12
THE VEGETATIVE SHOOT APEX IN 2EA MAYS .......................................
The Tunica ...................................................................................... 14
The Corpus • • • • • • • • • • 16
Leaf Initiation ............................................................................ 22
The A xillary Bud .......................................................... 25
Size of the Shoot Apex .......................................................... 31
THE SHOOT APEX IN RELATION TO CERTAIN MORPHOGENETIC STUDIES 37
THE FLORAL APEX
Introduction » • • • ................................................ 50
Transition from the Vegetative to the Floral Axis
in Zea .............................................................................. 61
Origin of the Floral Apex and the Floral Organs • 65
SUMMARY ................................................................................................................ 71
BIBLIOGRAPHY................................................................................................ 75
PLATES I - XII ................................................................................................
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INTRODUCTION
Any thorough understanding of the adult plant body depends on the
processes by which the foundations of the organs are laid down and the
steps by which they are developed* These studies necessarily begin with
the meristems. Recent trends in plant anatomy have placed a new emphasis
on these problems* The purpose of this study is to investigate the shoot
apex of Zea mays* The shoot apex of maize was selected for this study be­
cause the apex is relatively large and can be dissected out of the plant
with ease, and the apices can be oriented in the plane of distichism and
thus facilitate the study of the origin of the lateral organs.
The main objectives of this study are: (1) To study and interpret
the structure of the vegetative shoot apices in Zea mays in the light of
the tunica-corpus theory, and to study the origin of the leaves and ax­
illary buds from the terminal meristems. (2)To undertake in connection
with the study of the shoot apex, two morphogenetic problems involving
the size of the leaves and the size of the shoot apex from which the
leaves are produced. The first problem deals with the comparison of
greenhouse grown corn and field grown com; the second problem deals wi tb.
a genetic strain of maize in which the size of the leaf is determined by
one gene. (3) To study the transition of the vegetative apex to the in­
florescence or floral apex, the origin of the floral apices, and the
origin of the floral organs.
The shoot apex or terminal meristem was first observed by GASPER
F. WOLFF in 1759. He called this the "Punctum vegetationi3H, or grow­
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ing point. He correctly noted the importance of the apices in pro­
ducing apical growth, and recognized the method whereby the lateral or­
gans are produced'by the terminal meristem. In the early years of the
19th century emphasis was placed on the importance of individual cells.
NAGELI was especially influenced by the ceil theory. In 1845, and later
in 1858 and 1878, he put forth the theory that all organs and a ll tissues
of the shoot ootild be traced back to one cell situated at the apex of
the stem. His first studies were on various species of algae and moss­
es, and these were later extended to the lower vascular plants. In
many of the latte r an apical cell is present, and all cells can be
traced back to it (Wardlow, 1945; Golub, et a l., 1948). In his later
papers Nageli studied the high vascular plants and applied |iis apical
cell theory to them as well.
HOFMEISTER, who reported in 1851 on embryos in the gyranosperms
and lower plants, discovered the terminal cells in these and compared
them to the apical cells found by Nageli in mosses and Equisetum. In
later papers (1852, 1857, 1859) Hofmeister studied the embryos of dicoty­
ledons and monocotyledons, and here also he described in itial ceils situ­
ated at the apex of the embryo. In several cases, however, Hofmeister
found that growth proceeds from more than one in itial or apical cell, and
thus he disagreed with Nageli. In one paper (1857) he described a term­
inal cell in rye as a tetrahedral cell situated at the apex of the term­
inal meristem to which the origin of a ll cells could be traced.
After investigating 46 genera of angiosperms (including Zea), and
influenced by the previous work of Caspary (1858, 1859), Sanio (1864),
and Muller (1866), HANSTEIN brought forth in 1868 and 1870 his histogen
theory* He found in the shoot apex of embryos definite layers which
appeared early and maintained th eir independence during the entire life
of the plant* He called these layers histogens and considered them in­
dependent of one another. He assigned specific functions to each layer
and found one or more in itial cells in each. The histogens were des­
cribed as: (1) the dermatogen, which gives rise to the epidermal system
of the stem and the leaf and controls the outer part of the shoot apex;
(2) the periblem, which is made up of the underlying layer or layers of
cells and gives rise to the inner tissue of the leaf and to the cortex
of the stem; and (3) the plerome, which forms the central mass of the
shoot apex and gives rise to the pith and the procambial tissue of the
stem.
Following the work of Nageli, Hofmeister, and Hanstein, there ap­
peared many papers devoted to the study of the shoot apex in mono­
cotyledons, dicotyledons, gymnosperms, and the lower vascular plants.
The investigators reported that they found the apical cells of Nageli
or the histogens of Hanstein or that they failed to find either, and
they argued that a new interpretation of the terminal meristem was
necessary. In 1915 Herrig published a paper on Galium, Hippuris, and
Elodea in which he described and pictured the typical histogens as set
forth by Hanstein. And even as later as 1930 (Porterfield) the shoot
apex was interpreted according to Hanstein1s histogens.
After the turn of the century, interest in apical meristems waned
for two decades. An entirely new interpretation of the shoot apex,
based on the dynamics of growth, was advanced by SCHMIDT in 1924 and
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BUDIE in 1928. The outer peripheral layer or layers of the apex was re ­
ferred to as the tunica. The cells of this layer or layers divide only
anticlinally, except when lateral appendages are produced. The rest of
the apex is referred to as the Corpus; here the cells divide anticlinally,
periclinaily, and in various other directions, and the majority of them
are more or less irregularly arranged. The coipus produces growth in
volume, while the tunica forms the surface growth. The mode of cell di­
vision separates the two. Leaves form as "folds” produced on the flanks
of the apex by an increase in surface growth over the volume growth of
the corpus.
Since this new interpretation appeared, many studies made on
angiosperms, gymnosperms* and the lower vascular plants have resulted
in a new conception of the shoot apex. In this country especially, the
work on meristems has been stimulated by Foster’s early work and re­
views (1936, 1939). Several recent papers have appeared containing re­
views of the literature and of the different theories. These reviews
are by Foster (1941, 1949), Sifton (1944), Wardlow (1945), Eames and
MacDaniels (1947), Reeve (1948), and Majumdar (1945).
PREVIOUS WORK ON GRASS SHOOT APICES
In 1884 KORSCHELT published a paper dealing with the shoot apices
of gymnosperms and angiosperms; the latter included five grasses —
Saccharum officinarum, Festuca rubra, F. capillifolia, Panicum plicatum,
and Eulalia. In these grasses he described a single terminal cell at
the apex of the meristem, with the cells of the epidermis grouped around
it* He presented cross sections only and had nothing to say about the
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underlying tissues. The summit cell appears like an ordinaiy five-sided
cell of the tunica layer and seems to be in no way distinctly different
from the other tunica cells. Eorschelt was a supporter of Nageli*s
apical cell theory.
GROOM, in 1685, in studying the shoot apices of gymnosperms,
angiospezms, and the lower vascular plants, gave an exact interpretation
of his observations. He examined two grasses, Festuca and Fanicum. In
the cryptogams he was able to make out an apical cell; in the angiosperms
he identified the three histogens; and for the gymnosperms he concluded
that they represented intermediate stages. Nevertheless, because in
Festuca he could not make out a clear periblem, and plerome, he suggested
that perhaps they were not present — "I am in doubt whethe r in both
grasses a true periblem aid plerome is or is not presentn. Also he failed
to find an apical cell at the smtmit of the apices of these two grasses.
Accordingly he rejected both the apical cell theory and the histogen
theory and suggested that the summit growth must therefore be interpreted
from a new viewpoint.
DOULIOT in 1890 described 16 monocotyledons, 42 dicotyledons, and
22 gymnosperms, many of them well illustrated. He presented an excel­
lent review of previous wo rk and called attention to the fact that since
1852 the shoot apices of more than twenty different genera had been ex­
amined by more than twenty authors; yet there was little if any agreement
on the structure of the shoot apex. Douliot studied two grasses —
PhragBdtes communis and Baldingera (Fhalaris) arundinacea. He recognized
the three histogens in the apices of these grasses and named them the
epidermis, the cortex, and the central cylinder. The outer layers have
only perpendicular divisions. The second layer is often irregular; it
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may consist of one or more rows of cells* He located three in itia l c ells,
one for each layer, which behave as apical cells to perpetuate independ­
ently the three histogens.
The following year Douliot published a paper dealing entirely with
the shoot apex of grasses* The grasses he studied were Phragmites
communis, A1 ope cur us nigricans, Triticum sativum, Sorghum saccharum,
G-lyceria spectabilis, and Qryza sativa, and he mentions, without discussion
or illustrations, Panioum plicatum, Zea mays, and Baldingera arundinacea.
He also corrects his "error” of the previous paper, saying, "the stem of
the Gramineae grows by two cells, in which the terminal one generates the
epidermis and leaves; the subterminal cell generates the cortex and the
central cylinder". On the formation of the leaves he has this to say re­
garding Phragmites, Alopecurus, and Avena: "The cortex and the epidermis
take part in the formation of the portion of the leaf concrescent with
the stem, and the leaf grows by a single terminal cell”. Iiis drawings,
however, show the leaf originating entirely from the dermatogen layer.
MACMILLIAN, in 1892, presented a brief review or summary of
Douliot's work. Apparently Douliot was trying to compromise the apical
cell theory and the histogen theory. Although he does not deny the ex­
istence of stratified layers in the shoot apex, he nevertheless places
considerable emphasis on locating the apical cells which perpetuate each
layer. The idea that an apical ceil is necessary for the growth of the
shoot apex seems to have dominated the thoughts of that day; and even
recently Porterfield (1930) searched for apical cells for each histogen.
In 1893 PLOT presented the different ideas of the term inal
meristems as given by Hanstein, Russou, Flot, Van Tiegham, and others.
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H© explains that the dioot apex can he divided into different zones,
layers, in itials, etc., such as conjunctif meristem, endistem, desmogene,
and the use of terms of th is nature is reflected in recent work (Sharman,
1943, 1945). P lot's main contribution to the study of term inal meristems
was to point out that these different layers can be traced to two or more,
sometimes many, in itials rather than to one. The only grass that Plot ex­
amined was Holcus mollis (feather grass) of the tribe Aveneae. He shows
in a drawing three layers, the outer two of which quite obviously repre­
sent the tunica. However, he calls the second layer the cortex. He
points out a cell in the third layer as the f,in itia ln of the "vasculaire
meristem”.
BARHNETSKY, in 1897, studied cross sections of the shoot apex of
Bambusa arundinacea. In addition to an account of the origin of the
vascular tissue, he calls attention to the fact that the epidermal cells
of the terminal meristem divide tangentially to initiate the leaf and
that the tissues of the leaf come principally from the epidermis of the
shoot apex. The cross sections of the meristem show a single dermatogen
layer.
EUG-NQN (1924), using the tunica-corpus conception for the first
time, studied Dactylis glomerata and Melica altissim a. He found a single­
layered tunica overlying the corpus. He found periclinal divisions in the
tunica followed by divisions in the corpus initiating the leaves.
In 1930 PORTERFIELD published a study on the morphology of the shoot
apex in a bamboo, Phyllostachys nigra. Apparently he was unaware of
Buder's and Schmidt's work and the new conception of the shoot apex* He
states that he ’’was struck by the clear differentiation of the meristems"
as outlined by Hanstein. "It seems to me to be a very good illustration