A Handbook of Some South Indian Grasses

by K. Rangachari; C. Tadulinga Mudaliyar

The name rachis is given to the axis of the spike, raceme and panicle, whether the axis is the main one or of the branch. The rachis of the inflorescence is usually cylindrical. In some grasses it is zigzag as in Pennisetum cenchroides. It is very much flattened in Paspalum scrobiculatum, but somewhat trigonous in Digitaria sanguinalis. In very many grasses the rachis is continuous, but in a few cases it consists of internodes or joints which disarticulate at maturity. Many species of Andropogon have such jointed rachises. Sometimes the joints become greatly thickened and the surface hollowed out, the spikelets fitting in the cavities as in Rottboellia and Manisuris.

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 coromandelianus the branches are verticillate. In both these grasses 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 coromandelianus. These glands secrete a viscid juice at the time of anthesis.

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 grasses. 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 homologous with the prophyllum which it very much resembles. The prophyllum is usually found in the branches of grasses, but it is not confined to grasses alone. It occurs in the branches of some species of Commelina.

The spikelets vary very much in their structure. The spikelets in grasses 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 unisexual 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 Panicum 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 Panicum. 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.