The epidermis covering the leaves consists of elongated cells with plane or sinuous walls, various kinds of short cells intercalated between the ends of long cells, motor-cells and stomata. Hairs of different sorts occur as outgrowths of the epidermis. The roughness of the surface of the leaves of gra.s.ses is due to the presence of very minute short hairs borne by the epidermis. In most cases these short hairs are found in regular rows. Although the epidermis is more or less even in the leaves of several gra.s.ses such as _Panic.u.m repens_, _P. flavidum_ and _Eriochloa polystachya_, it is wavy or undulating in the leaves of a few gra.s.ses. For example, the upper epidermis in the leaves of _Panic.u.m fluitans_ is undulating as it follows the contour of the ridges and furrows.
The epidermal cells have even surfaces in the leaves of most gra.s.ses but in some they bulge out. In the leaves of _Panic.u.m flavidum_ the cells of the lower epidermis are quite even, whilst those of the upper epidermis bulge out. The cells of both the upper and the lower epidermis are distinctly bulging out in the leaves of _Panic.u.m colonum_. In _Panic.u.m fluitans_ the cells of the upper epidermis bulge out so much as to form distinct papillae.
The free surface of the epidermis is more or less cutinised in the leaves of all gra.s.ses. In some leaves the cuticle is very thick and even papillate as in the leaves of _Aristida setacea_ and _Panic.u.m repens_ whilst in others it is very thin, as in the leaves of _Panic.u.m colonum_ and _P. fluitans_. Cutinisation is rather prominent in the leaves of gra.s.ses growing under dry conditions and it is less p.r.o.nounced in mesophytic gra.s.ses.
[Ill.u.s.tration: Fig. 64.--Transverse section of a portion of the leaf of Panic.u.m colonum. 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
[Ill.u.s.tration: Fig. 65.--Transverse section of a portion of the leaf of Panic.u.m fluitans. 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
As regards size, the epidermal cells overlying the sclerenchyma are small and those lying over parenchyma are larger. Amongst the larger cells some may be motor-cells. The stomata occur in regular rows between the vascular bundles and they are quite characteristic of gra.s.ses. They are more or less similar in structure in all gra.s.ses. In the leaves of many gra.s.ses stomata are found in both the upper and the lower epidermis and they are confined to the lower epidermis in a few gra.s.ses only.
The motor-cells vary very much both as regards their shape and position.
In some leaves as in the leaves of the gra.s.s _Panic.u.m flavidum_ the motor-cells are confined to the midrib on the upper surface.
The epidermal cells of this leaf are large and uniformly round. (See figs. 66 and 67.)
[Ill.u.s.tration: Fig. 66.--Transverse section of a leaf of Panic.u.m flavidum. 20
1. Vascular bundle; 2. sclerenchyma; 3. motor-cells.]
[Ill.u.s.tration: Fig. 67.--Transverse section of the leaf of Panic.u.m flavidum. 150
1. Motor-cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
In the case of most gra.s.ses the motor-cells are found in groups of three, four or five between the vascular bundles. The central motor-cell is usually the largest and it is somewhat obovate in shape in a transverse section of the leaf. In the leaves of _Panic.u.m javanic.u.m_ and _Eriochloa polystachya_ there are three or four motor cells in the group and the group consists of four, five or rarely six motor cells in the leaves of _Eragrostis Willdenoviana_. When there are distinct furrows between ridges these cells lie in the furrows and they are many in number. In the leaves of _Panic.u.m repens_ there are five to seven motor-cells in the furrows and the single row of cells stretched between the motor-cells and the lower epidermis in the furrow consists of more or less clear cells with spa.r.s.ely scattered small chlorophyll grains.
(See fig. 52.) The motor-cells occupying the furrows in the leaves of _Aristida setacea_ are more in number than in _Panic.u.m repens_ and are of a different shape. All the cells lying in the furrow between the motor-cells and the sclerenchyma are clear cells free from chlorophyll grains.
Although the motor-cells differ in shape from the ordinary epidermal cells in most gra.s.ses, there are, however, a few gra.s.ses in which the motor-cells do not differ very much from the epidermal cells except in size. For example, in the leaves of _Panic.u.m colonum_ the motor-cells are just like the ordinary epidermal cells in shape but are larger. (See fig. 64.)
Motor-cells are usually confined to the upper epidermis, but they may also be found in the lower epidermis. In the leaves of _Pennisetum cenchroides_ motor-cells are found in both the upper and the lower epidermis, the group in the upper epidermis alternating with that in the lower.
[Ill.u.s.tration: Fig. 68.--Transverse section of a portion of the leaf of Pennisetum cenchroides. 100
1. Motor-cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
CHAPTER V.
CLa.s.sIFICATION.
The family Gramineae is usually divided into two series taking into consideration the presence or absence of a joint in the pedicel or rachis, the number of flowers in the spikelet and the position of the fertile flower. All the species in which there is a joint just below the spikelet, in the pedicel, in the rachis, or at the base of a cl.u.s.ter of spikelets come under one series =Panicaceae=. The spikelets of the gra.s.ses coming under this series, when mature, fall away singly by themselves, or with their pedicels, or in groups with portions of the rachis. The spikelets are all similar and consist of usually four glumes. Each spikelet contains a single perfect flower and sometimes in addition a staminate flower just below the perfect flower. In this series the tendency for imperfection is always confined to the lower flowers, the terminal flower alone being perfect. For inclusion under this series the gra.s.s plant should have both the characters, articulation and position of the flower as mentioned above.
The second series =Poaceae= includes those gra.s.ses in which the spikelets are one to many-flowered and continuous with their pedicels. But the rachilla of the spikelet may be jointed just above the empty glumes or between the flowering glumes. The complete flower is the lowest and the tendency for imperfection is in the upper flowers.
Of the two series the Panicaceae appears to be more highly developed than the Poaceae.
KEY TO TRIBES.
Series I.--Panicaceae.
A. Rachis of inflorescence not jointed.
Spikelets 2-flowered; upper flower bis.e.xual and lower male or neuter; the first glume the smallest I. Paniceae.
Spikelets 1-flowered;
Spikelets articulate on their pedicels and falling away from them; flowers bis.e.xual and usually with six stamens II. Oryzeae.
Spikelets falling away with their pedicels; flowers bis.e.xual or rarely imperfect III. Zoysieae.
B. Rachis of inflorescence usually jointed.
Spikelets usually binate (3-nate at the top), pairs of spikelets alike or dissimilar; empty glumes larger and the flowering glumes smaller, hyaline, the fourth glume awned or reduced to an awn IV. Andropogoneae.
Series II.--Poaceae.
A. Rachilla produced or not beyond the flowering glume.
Spikelets 1-flowered, with three glumes; first and second empty, third flowering and awned; rachilla jointed V. Agrostideae.
Spikelets 1- or more-flowered, biseriate and secund on an inarticulate spike or on the spiciform branches of a slender panicle; flowers all or the lower only bis.e.xual VI. Chlorideae.
B. Rachilla produced beyond the uppermost flowering glume and articulate.
Spikelets 2- or more-flowered, pedicelled, rarely sessile, in effuse, contracted or rarely spiciform panicles VII. Festucaceae.
Spikelets 1- or more-flowered, sessile, 1- to 2- or more-seriate on the rachis of a simple spike, or partially sunk in cavities of the same. Glumes awned or not, first and second glumes are opposite or subcollateral, persistent or separately deciduous; first glume minute or absent VIII. Hordeae.
CHAPTER VI.
Series I--Panicaceae.
TRIBE I--PANICEae.
This is a fairly large and important tribe flourishing mostly in the warm regions and the tropics. It is very well represented in South India and fifteen genera are met with.
The inflorescence varies very much within this tribe and consists of spikes, racemes and panicles. The spikelets are usually four-glumed and contain one terminal perfect flower and a staminate or neutral flower below. But in the genus Isachne both the flowers are perfect. In some gra.s.ses the spikelets contain only staminate or pistillate flowers. In Coix and Polytoca the plant bears both male and female spikelets in the same inflorescence, but in Zea on the same plant they occur as distinct inflorescences. The littoral gra.s.s Spinifex is dioecious.