THE VEGETATIVE ORGANS.
Grasses vary very much in their habit. Some grasses grow erect forming
tufts and others form cushions with the branches creeping along the
ground. (See figs. 5 and 6.) We usually find all intermediate stages
from the erect to the prostrate habit. Underground stems such as stolons
and rhizomes occur in some grasses. Grasses of one particular species
generally retain the same habit but this does not always hold good. For
example _Tragus racemosus_ grows with all its branches quite prostrate
in a poor, dry, open soil. If, on the other hand, this happens to grow
in rich soils, or amidst other plants or grasses, it assumes an erect,
somewhat tufted habit. _Andropogon contortus_ and _Andropogon pertusus_
are other grasses with a tendency for variation in habit. Plants that
are usually small often attain large dimensions under favourable
conditions of growth. Ordinarily the grass _Panicum javanicum_ grows
only to 1 or 2 feet. (See fig. 1.) The same plant in a good rich soil
grew to about 6 feet in four months. (See fig. 7.)
[Illustration: Fig. 5.--Eleusine aegyptiaca.]
Some grasses are annual while others are perennial. It is often
difficult to determine whether a certain grass is annual or perennial.
But by examining the shoot-system this can be ascertained easily. In an
annual all the stems and branches usually end in inflorescences and they
will all be of the same year. If, on the other hand, both young leafy
branches and old branches ending in inflorescences are found mixed, it
must be a perennial grass. The presence of the remains of old leaves,
underground stolons and rhizomes is also evidence showing the perennial
character of the plant.
Grasses are eminently adapted to occupy completely large areas of land.
They are also capable of very rapid extension over large areas, on
account of the production of stolons, rhizomes and the formation of
adventitious roots.
=The root-system.=--The root-system of grasses is very striking in its
character. In most grasses, especially in erect ones, several roots all
of about the same diameter arise in a dense tuft from nearly the same
level and from the lower-most nodes of the stems. The roots are all thin
and fibrous in the vast majority of these plants, and they are tough and
wiry only in a few cases such as in the case of the roots of _Pennisetum
cenchroides_, _P. Alopecuros_, _Ischaemum pilosum_ and _Andropogon
Schoenanthus_.
On a close examination it will become evident that all the roots of a
grass plant are adventitious. Inasmuch as the growth of the primary root
is soon overtaken by other roots growing from the stem, all the roots
happen to be of the same size. Roots arise from the nodes just above the
insertion of the leaf, and they grow piercing the leaf-sheath.
[Illustration: Fig. 6.--Panicum Crus-galli.]
Grasses in which stolons and prostrate branches occur have, in addition
to the usual radiating crown of roots at the base, aerial roots growing
out of the upper nodes of the branches and fixing them to the soil. Such
roots become supporting or prop roots and are particularly conspicuous
in several stout tall grasses such as _Andropogon Sorghum_, _Zea Mays_
and _Pennisetum typhoideum_. (See figs. 8 and 9.)
All the roots bear branch-roots which originate from the inner portion
of the mother roots in the usual manner. The character and the extent of
the development of the root-system is to a large extent dependent upon
the nature of the soil and its moisture content. In light dry soils
roots remain generally stunted and in well drained rich soils they
attain their maximum development. In clayey soils roots penetrate only
to short distances. When the soil is rich and sandy roots go deeper and
extend in all directions. The root-systems of most grasses are
superficial and so are best adapted for surface-feeding.
[Illustration: Fig. 7.--Panicum javanicum.]
=The shoot-system.=--The shoot-system varies with the duration of the
life of the plant. In annual grasses stems are in most cases erect and
even if they are not entirely so they become erect at the time of
flowering. They are attached to the soil by a tuft of fibrous roots
arising from the base of the stems. But in perennials in addition to
erect branches, creeping branches, stolons and rhizomes may occur.
[Illustration: Fig. 8.--Prop roots of Andropogon Sorghum.]
[Illustration: Fig. 9.--Aerial roots of Ischaemum ciliare.]
The stem is either cylindrical or compressed and consists of nodes and
internodes. In most grasses the internodes are usually hollow, the
cavity being lined by the remains of the original pith cells. However,
there are also grasses in which the stems remain solid throughout. In
many grasses the basal portions of stems are more leafy and the
internodes are short, but in the upper portions the internodes become
longer separating the leaves one from the other.
In young shoots the leaves grow much faster than the internodes and
consequently internodes remain small, and leaves become very
conspicuous. The youngest portions of the shoots are by this means
always well protected by the surrounding leaf-sheaths. As soon as leaves
have grown fully, the internodes begin to elongate rapidly separating
the leaves. At first growth in length takes place throughout its length
in the internode and when it gets older this elongation ceases. But,
however, the lower portion of the internode close to the node and which
is enclosed by the leaf-sheath retains its power of growth for a
considerable time.
Branches arise from the axils of leaves and when a considerable number
of the axillary buds, especially from the lower nodes, develop into
branches the plant becomes tufted in habit. In most grasses branches
grow upwards through the sheath and emerge at its mouth as aerial
branches. Such branches are called =intravaginal= branches or stems. But
in some grasses axillary buds, instead of growing straight up through
the sheath, pierce the leaf-sheath, come out and then they grow out as
branches. This may be seen in the underground stolons of _Panicum
repens_ and in the ordinary aerial branches of _Arundo Donax_. Branches
that pierce through the sheaths are called =extravaginal= branches. (See
fig. 10.)
[Illustration: Fig. 10.--Extravaginal shoots of 1. Panicum repens and 2.
Arundo Donax.]
[Illustration: Fig. 11.--Nodes.
1. Glabrous node; 2. bearded node; 3. node cut longitudinally.]
The nodes are in most cases very conspicuous and they are often found
swollen. However, it must be remembered that the enlargement at the node
is not due to the increase in size of the actual node, but due to growth
in thickness of the base of the leaf-sheath. (See fig. 11-3.) Nodes may
be pale or coloured, glabrous, hairy or bearded with long hairs. When
the stem is erect the nodes are short and of uniform size all round.
But, if the stem is bent down or tipped over by accident, the nodes
begin to grow longer on the lower side until a curvature sufficient to
bring the stem to the erect position is formed and then it ceases to
grow.
As already noted some perennial grasses have creeping stems and stolons,
while others may have rhizomes. The grass _Cynodon dactylon_ develops
several underground stolons which are covered with white scale leaves
and whose terminal buds are hard and sharp so that they may be able to
make their way through the soil. The rhizomes when continuous and
elongated are usually sympodia formed by the lower portions of the
aerial shoots. The aerial shoot comes into the air and its lower portion
is continued by a branch arising from a lower leaf axil beneath the
soil.
=The leaf.=--Leaves are two-ranked and alternate, and very often they
become crowded at the lower portions of the shoots so as to form basal
tufts, though they are farther apart in the upper portions of these
shoots. Three distinct kinds of leaves are met with in grasses. First,
we have the fully formed foliage leaves so characteristic of grasses.
These are most conspicuous and are formed in large numbers.
The other two kinds of leaves are neither so conspicuous nor so numerous
as the foliage leaves. At the base of shoots occur abortive leaves which
are really rudimentary sheaths. These are called =scales=. The third
kind of leaf is a modified structure called the =prophyll= or
=prophyllum=. (See fig. 12.) It is the first leaf occurring in every
branch on the side next to the main shoot and it is a two-keeled
membranous structure resembling somewhat the palea found in the
spikelets of grasses. The portion of the prophyll between the keels is
concave due to the pressure of the main stem, while the sides beyond the
keels bend forward clasping the stem.
[Illustration: Fig. 12.--Prophylla.
A. A branch with its prophyllum; B. prophyllum; C. section of the
prophyllum.]
The ordinary foliage leaves of grasses consist of the two parts, the
flat expanded upper portion called the =blade= and the lower part called
the =sheath= that encircles the stem above the node from which it
arises. The leaf-sheaths usually fit close to the stem, but they may
also be loose or even inflated. Though the leaf-sheath surrounds the
internode like a tube, it is not a closed tube. It is really a flat
structure rolled firmly round the stem with one edge overlapping the
other. In most cases it is cylindrical and it may be compressed in a few
cases. Occasionally it may have a prominent ridge or keel down its back.
The sheath may be glabrous or hairy, smooth or striate externally, and
the outer margin is often ciliate. In a few grasses the sheaths become
coloured especially below or on the side exposed to the sun.
[Illustration: Fig. 13.--Ligules of 1. Oryza sativa; 2. Panicum
javanicum; 3. Andropogon Schoenanthus; 4. A. contortus.]
[Illustration: Fig. 14.--Shapes of leaf-blades.
1, 7 and 8. Lanceolate; 3 and 6. lanceolate-linear; 2 and 5. linear; and
4. ovate.]
The =ligule= is a structure peculiar to grasses and it varies very much.
In some grasses it is a distinct membrane narrow or broad, with an even,
truncate or erose margin, or finely ciliate. Very often it is only a
line or fringe of hairs, whilst in some it may be entirely absent as in
the leaves of _Panicum colonum_. When it is a membrane it may be broad
and oblong, ovate and obtuse, or lanceolate and acute. (See fig. 13.)
The function of the ligule is probably to facilitate the shedding of
water which may run down the leaf, and thus lessen the danger of rotting
of the stem which is sure to follow, if the water were to find its way
into the interior of the sheath. Sometimes, in addition to the ligule,
other appendages may be present in grass leaves as in _Oryza sativa_.
Such outgrowths are called =auricles= or =auricular outgrowths=. (See
fig. 13.)
The leaf-blade is well developed in the foliage leaves and in most cases
it follows directly on the sheath. But in bamboos and some species of
Ischaemum there occurs a short petiole or stalk between the leaf-blade
and the sheath. The sheath corresponds morphologically to the leaf base
of a leaf of other flowering plants.
[Illustration: Fig. 15.--Margins of leaves.
1 and 2. Finely serrate; 3. glandular; 4 and 5. very minutely serrate;
6. very minutely serrate and ciliate.]
In grasses the leaf-blades usually grow more in length than in any other
direction and there is no limit to the length they may attain. Some
grasses have very short leaves, others very long ones. The leaf-blade in
most grasses is more or less of some elongated form, such as linear,
linear-lanceolate, lanceolate, etc. (See fig. 14.) In a few grasses the
leaf-blade is ovate, but this is a rare condition. Therefore, in noting
the general shape of the leaf-blade the relation of the length to the
breadth, the amount of tapering towards the apex and base and the nature
of the apex should be considered.
The veins in the leaf-blade can usually be seen on holding the leaf up
to the light. All the veins run parallel. In most cases the midrib is
prominent and in some cases there may be also a distinct keel. Amongst
the veins running through the leaf-blade some are large and prominent,
while others are small and not conspicuous. On account of this
disparity, very often, ridges and furrows become prominent on the upper
or lower, or on both the surfaces of the leaf-blades. Generally the two
surfaces of the leaf-blade are distinct, and they may be glabrous or
hairy. In most grasses the surfaces are rough or scabrid to the touch
owing to the presence of regular rows of exceedingly fine sharp pointed
minute hairs.
The apex of the blade is generally sharp and pointed, acute or
acuminate, or sometimes it may be drawn to a very fine point by gradual
tapering. Blunt or obtuse tips are not altogether absent, but it is not
a common feature. The leaf-blades in _Panicum colonum_ and in some
species of Andropogon are rounded or obtuse at the apex.
The margins of the leaf-blade are somewhat hyaline and they may be
perfectly even or cut into serrations of fine teeth in various ways.
(See fig. 15.) In addition to these minute teeth, there may be long or
short cilia. Sometimes the margins are glandular as in _Eragrostis
Willdenoviana_ and _Eragrostis major_.
The base of the leaf may be narrower, broader than, or about the same as
the breadth of the leaf-sheath. It may be rounded, amplexicaul or
narrowed. At the base and just above the ligular region there will
always be a white distinct zone in the lamina of all grasses called the
collar. This collar varies in length and breadth according to the
species of grass.
[Illustration: Fig. 16.--Transverse section of leaf-buds.
A. Conduplicate; 1, 2 and 3. leaf-sheaths; 4 and 5. leaf-blades. B.
Convolute; 1 and 2, leaf-sheaths; 3 and 4. leaf-blades.]
In young shoots all the leaf-blades are usually found folded at the
terminal portions. In most cases the leaf-blade is rolled up inwards
from one end to the other so that one margin is inside and the other
outside. This folding is termed =convolute=. This is the kind of folding
that is found in most grasses. However, there are some grasses such as
_Eleusine aegyptiaca_ and _Chloris barbata_, in which the folding is
different. In these grasses the laminas are folded flat on their midribs
so that each half of the blade is folded flat on the other, the inner
surfaces being in contact. The leaves are said to be =conduplicate= in
this case. When the leaves are conduplicate the shoots are more or less
compressed. (See fig. 16.)