A Handbook Of Some South Indian Grasses Part 2

In panicles, especially when they are diffuse, the primary branches may be disposed irregularly or in verticils on the main axis. For example in the panicle of _Eragrostis Willdenoviana_, the branches are irregularly disposed, whereas in _Sporobolus coromandelia.n.u.s_ the branches are verticillate. In both these gra.s.ses fleshy cushions are developed in the axils of the branches. These swellings help to spread out the branches especially at the time of anthesis. The branches at the top spread out earlier than those below.

Sometimes at the base of the rachises, main or secondary, glandular streaks are seen as in the rachises of _Sporobolus coromandelia.n.u.s_.

These glands secrete a viscid juice at the time of anthesis.

[Ill.u.s.tration: Fig. 17.--The Spikelet of Dinebra arabica.

1 and 2. Empty glumes; 3, 4, 5, and 6. flowering glumes with flowers.]

The =spikelet= may be considered as a specialised branch consisting of a short axis, the =rachilla= bearing a series of modified bracts, the =glumes=, the lower pair being empty but the others bearing flowers in their axils. The glumes are two-ranked and imbricating. As a type for the spikelet that of an Eragrostis or Dinebra may be chosen. (See fig.

17.) In this spikelet the rachilla bears a number of glumes alternating and imbricating. The first two glumes at the base of the spikelet do not bear any flowers and so these two glumes are usually called empty glumes. This is the case in almost all the species of gra.s.ses. The third and the subsequent glumes are regularly arranged on the slender rachilla alternately in two rows. In the axils of each of these glumes there is a flower, except perhaps in the topmost glume. The flower is usually enclosed by the glume and another structure found opposite the glume and differing very much from the glume. This is the =palea=. It is attached to the axis of the flower and its back is towards the rachilla.

Generally there are two nerves in a palea and its margins are enclosed within those of the glume. The palea is h.o.m.ologous with the prophyllum which it very much resembles. The prophyllum is usually found in the branches of gra.s.ses, but it is not confined to gra.s.ses alone. It occurs in the branches of some species of Commelina.

The spikelets vary very much in their structure. The spikelets in gra.s.ses of several genera consist of only four glumes. As usual the first two glumes are empty and the remaining two are flower-bearing glumes. Both these glumes may have perfect flowers as in Isachne or the terminal one may contain a perfect flower, the lower having either a staminate flower or only a palea. Very often the spikelets are unis.e.xual and the male and female spikelets may be on the same plant as in _Coix Lachryma-Jobi_ and _Polytoca barbata_, or they may be on different plants as in _Spinifex squarrosus_.

The glumes of a spikelet are really modified bracts and some differentiate the flowering glumes from the empty ones, by giving them different names. The first two empty glumes are called glumes by all agrostologists. Some in Europe call the flowering glume lower palea to distinguish it from the real palea which they call the upper palea. Some American Authors have recently adopted for the flowering glume the term =lemma= introduced by Piper.

Considerable variation is met with in the case of the empty glumes.

Generally they are unequal, the first being smaller. Very often the first glume becomes very small and it may be altogether absent. In some species of Panic.u.m the first glume is very small, in Digitaria it is very minute and in Paspalum and Eriochloa it is entirely suppressed. The flowering glumes are generally uniform when there are many. In the spikelet having only four glumes the fourth glume differs from the others mainly in texture. Instead of being thin and herbaceous it becomes rigid and hard, smooth or rugose externally as in Panic.u.m.

Flowering glumes instead of being like empty glumes, become very thin, shorter and hyaline in Andropogon. Sometimes the flowering glumes are awned. All of them may be awned as in Chloris or only the fourth glume as in Andropogon.

The palea is fairly uniform in its structure in many gra.s.ses, but it is also subject to variation. It becomes shorter in some and is absent in others. Instead of having two nerves, it may have one and rarely more than two. The palea can easily be distinguished from the glume, because its insertion in the spikelet is different from that of the glume.

[Ill.u.s.tration: Fig. 18.--Flower of Chloris.

1. lodicules; 2. stamens; 3. ovary.]

The =lodicules= are small organs and they are the vestiges of the perianth. In most gra.s.ses there are only two, but in Ochlandra and other bamboos we meet with three lodicules. There are also some species with many lodicules. In shape they are mostly of some form referable to the cuneate form. They are of somewhat elongated form in Aristida and Chloris. The function of the lodicules seems to be to separate the glume and its palea so as to enable the stamens to come out and hang freely at the time of anthesis. So it is only at the time of the opening of the flowers that the lodicules are at their best. Then they are fairly large, fleshy and thick and conspicuous. In the bud stage they are usually small and after the opening of the flower they shrivel up and are inconspicuous. There are also species of gra.s.ses in which the lodicules are not found.

The =stamens= are three in number in the majority of gra.s.ses and six are met with in Leersia, Hygrorhiza and Bamboos. Each stamen consists of a very delicate long filament and an anther basifixed to the filament. But as the anthers are long and the connective is reduced to its minimum, they appear as if versatile when the stamens are out. When there are three stamens one stands in front of the flowering glumes and the other two in front of the palea, one opposite each edge of the palea. The relative positions of the parts of the floret are shown in the floral diagrams. (See figs. 18 and 19.)

[Ill.u.s.tration: Fig. 19.--Floral diagrams.

The first is that of Chloris, second of Panic.u.m and the third of Oryza.]

The =pistil= consists of an ovary and two styles ending in plumose stigmas. The ovary is 1-celled and 1-ovuled. It is one carpelled according to the views of Hackel and his followers and there are also some who consider it as 3-carpelled because of the occurrence of three styles in the pistil of some bamboos.

The =rachilla= is usually well developed and elongated in many-flowered spikelets, while in 1-flowered spikelets it remains very small so that the flower appears to be terminal. It often extends beyond the insertion of the terminal flower and its glume, and then lies hidden appressed to the palea. This may be seen in the spikelets of the species of Cynodon.

This prolonged rachilla sometimes bears a minute glume, which is of course rudimentary. Usually the glumes are rather close together on the rachilla so that the internodes are very short; but in some gra.s.ses, as in _Dinebra arabica_, the glumes are rather distant and so the internodes are somewhat longer and conspicuous. In some species of Panic.u.m the rachilla is jointed to the pedicel below the empty glumes, whereas it is articulated just above these glumes in _Chloris barbata_.

Sometimes the rachilla is articulated between the flowers. This is the case in the spikelet of _Dinebra arabica_.

Pollination in most gra.s.ses is brought about by wind, though in a few cases self-pollination occurs. The terminal position of the inflorescence, its protrusion far above the level of the foliage leaves, the swinging and dangling anthers, the abundance of non-sticking pollen and the plumose stigmas are all intended to facilitate pollination by wind. Furthermore the stamens and the stigmas do not mature at the same time. In some gra.s.ses the stamens mature earlier, (=protandry=) while in others the stigmas protrude long before the stamens (=protogyny=). As the result of the pollination the ovary developes into a dry 1-seeded indehiscent fruit. The seed fills the cavity fully and the pericarp fuses with the seed-coat and so they are inseparable. Such a fruit is termed a =caryopsis= or =grain=. Though in the vast majority of gra.s.ses the pericarp is inseparable, in a few cases it is free from the seed-coat as in _Sporobolus indicus_ and _Eleusine indica_.

[Ill.u.s.tration: Fig. 20.--Longitudinal section of a portion of the grain of Andropogon Sorghum. 280 P. Pericarp; Sc. seed-coat; A. aleurone layer; E. endosperm; S. scutellum; Rs. root-sheath; Rc. root-cap; R.

radicle; Pl. plumule; G. growing point.]

[Ill.u.s.tration: Fig. 21.--A portion of the section of the grain of Andropogon Sorghum. 500 P. pericarp; I. seed-coat; A. aleurone layer; S. starch.]

The caryopsis consists of an embryo on one side at the base and the endosperm occupies the remaining portion. The embryo can be made out on the side of the grain facing the glume, as it is outlined as an oval area. On the other face of the grain which is towards the palea, the hilum is seen at the base. The grain varies in shape considerably. It may be rounded, oval, ellipsoidal, narrow and cylindrical, oblong terete or furrowed. There is considerable variation as regards the colour also.

The =embryo= consists of an =axis= and a =scutellum=. The axis, which is differentiated into the plumule directed upward and the radicle downward, is small and straight and it is covered more or less by the edges of the scutellum. The scutellum is attached to the axis at about its middle and its outer surface is in contact with the endosperm. This is an important organ as its function is to absorb nourishment from the endosperm during germination. The scutellum is considered to represent the first leaf or cotyledon. The endosperm consists mostly of starch.

Just outside the endosperm and within the epidermis lies a layer of cells containing much proteid substance. This layer is called the =aleurone layer=. (See fig. 21.) As an ill.u.s.tration of the caryopsis, the grain of Andropogon Sorghum may be studied. All the structural details are shown in fig. 20 which is a longitudinal section of the grain.

The primary axis of the embryo is enclosed by a closed sheath both above and below. The sheath which envelopes the radicle is called =coleorhiza= and that of the plumule, =pileole= or =germ-sheath=.

CHAPTER IV.

HISTOLOGY OF THE VEGETATIVE ORGANS.

The shoots and roots of gra.s.ses conform in their internal structure to the monocotyledonous type. In all gra.s.ses numerous threads are found running longitudinally within the stem and some of these pa.s.s into the leaves, at the nodes, and run as nerves in the blades of the leaves.

These threads are the vascular bundles. The rest of the tissue of the stem and leaves consists of thin-walled parenchymatous cells of different sorts.

The general structure of these bundles is more or less the same in all gra.s.ses. A vascular bundle consists of only xylem and phloem, without the cambium, and so no secondary thickening can take place in the stems of gra.s.ses. Such bundles as these are called =closed vascular bundles= to distinguish them from the dicotyledonous type of vascular bundles which are called =open vascular bundles= on account of the existence of the cambium.

[Ill.u.s.tration: Fig. 22.--Transverse section of a vascular bundle. 250

1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. phloem or sieve tubes; 5. sclerenchyma.]

The component parts and elements of which the vascular bundles in gra.s.ses are composed may be learnt by studying the transverse and longitudinal sections of these bundles in any gra.s.s. The cross and longitudinal sections of a vascular bundle of the stem of _Pennisetum cenchroides_, are shown in figs. 22 and 23. In the figure of the transverse section the two large cavities indicated by the number 3 and the two small circular cavities with thick walls lying between the larger ones and indicated by the numbers 1 and 2 are the chief elements of the xylem.

By looking at the longitudinal section it is obvious that these elements are really vessels, the larger being pitted and the smaller annular and spiral vessels. These vessels together with the numerous small thick-walled cells lying between the pitted vessels const.i.tute the xylem. Just above the xylem there is a group of large and small thin-walled cells. This is the phloem and it consists of sieve tubes and thin-walled cells. All round the xylem and the phloem there are many thick-walled cells. These are really fibres forming the =bundle-sheath=.

On account of this bundle-sheath the bundles are called =fibro-vascular bundles=.

[Ill.u.s.tration: Fig. 23.--Longitudinal section of a vascular bundle.

250

1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. sieve tubes or phloem; 5. sclerenchyma.]

[Ill.u.s.tration: Fig. 24.--Transverse section of a portion of the stem of Rottboellia exaltata. 70

1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]

=Structure of the stem.=--The stem of a gra.s.s consists of a ma.s.s of parenchymatous cells with a number of fibro-vascular bundles imbedded in it, and it is covered externally by a protective layer of cells, the epidermis. The stem is usually solid in all gra.s.ses in the young stage, but as it matures the internodes become hollow in many gra.s.ses and they remain solid in a few. In the internodes the fibro-vascular bundles run longitudinally and are parallel, but in the nodes they run in all directions and form a net work from which emerge a few bundles to enter the leaves. So far as the broad general features are concerned, the stems of many gra.s.ses are more or less similar in structure. However, when we take into consideration the arrangement of bundles, the development and arrangement of sclerenchyma, every species of gra.s.s has its own special characteristics. And these are so striking and constant that it may be possible to identify the species from these characters alone.

We may take as a type the stem of _Rottboellia exaltata_. This stem is somewhat semi-circular in transverse section and it is almost straight and flat in the front (the side towards the axillary bud). The peripheral portion of the stem becomes somewhat rigid and thick due to the aggregation of vascular bundles, some small and others large. The outermost series of bundles consisting of small and larger bundles are in contact with the layers of the cells lying just beneath the epidermis and these cells are also thick-walled. A few are away from these being separated by three or four layers of cells from the peripheral bundles.

In all these vascular bundles the bundle-sheath is very strongly developed all round and is very much developed especially at the sides.

It is this great development of sclerenchyma that makes the outer portion of the cortex hard. Within the ground tissue are found a number of vascular bundles scattered more or less uniformly. These bundles have no continuous bundle-sheaths but have instead groups of fibres at the sides and in front of the phloem. The cavities near the annular vessels are somewhat larger and conspicuous in these bundles.

[Ill.u.s.tration: Fig. 25.--Transverse section of the stem of Pennisetum cenchroides. 20]

The epidermal cells are all thickened very much and the outer layer is cutinized and impregnated with silica. This is the case in the epidermis of the stems and leaves of most gra.s.ses. (See fig. 24.)

In order to give a general idea of the variations in the structure of the stem in gra.s.ses a few examples are chosen and the details of the structure of the stems of these gra.s.ses are dealt with here.

[Ill.u.s.tration: Fig. 26.--Transverse section of a portion of the stem of Pennisetum cenchroides. 70