Grass Morphology II

 Spikelet

           

Spikelet of Arundinella pumila (Hochst. ex A. Rich.)  Steud.

Spikelet is a basic unit in inflorescence, it comprises Glumes, Lemmas, Paleas and florets. Spikelet shows great variation in structure and arrangement, based on this, they are placed in different tribes. Most of the grasses have only one type of spikelet, but some shows exceptions; some are paired or solitary. Genera like Digitaria, Phacelurus, Saccharum etc. have only one type of spikelets, they are paired. Some of the genera like Sehima, Dichanthium and Heteropogon have paired and dissimilar spikelets . Grasses like Iseilema, Themeda, have three types of spikelets, involucral spikelet (Male), Sessile (bisexual) and Pedicelled (Male). In Pennisetum and Setaria, the spikelets are subtended by involucral bristles. In Zea, Coix and Chionachne, male and female spikelets are arranged in same or in different inflorescence. Spikelets of  Oryza, Leersia and Sporobolus, have only one floret. Species belonging to tribes Paspaleae, Andropogoneae, Paniceae and Isachneae have two florets either both fertile or one fertile and other sterile or male. Genera like Eragrostis, Tripogon and Eragrostiella have multiple florets.

Glumes

            Glumes are modified bracts arranged distichously at the base of the florets. Generally differentiated as Lower glumes and Upper glumes. In some genera like Oryza, Hygroryza and Leersia, they are either reduced into a scale or absent completely. Digitaria and Paspalum species have solitary glume. In Andropogoneae, glumes are dissimilar in size and texture; lower glumes are generally crustaceous and sculptured, grooved, wrinkled, pitted or provided with appendages. In Andropogoneae, glumes are equal to the size of the spikelets, whereas in Paniceae, Pooideae, Chloridoideae, glumes are smaller in size. In Oplismenus and Chrysopogon glumes are awned. Ornamentation, nervation, texture, keeled nature, termination of glumes are key features to distinguish grass taxa. (Photo plates 20 & 21)

            Lemmas are modified bracts bearing flowers. In genera like Oryza, Hygroryza and Leersia, the lemma is solitary. In Eragrostiella, Eragrostis and Tripogon, lemmas are many which are mostly similar. In Chloris and Centotheca, lemmas are multiple in number but dissimilar. In Paniceae, lemmas are 2, one lower male or sterile and upper bisexual. For protection of grain, upper lemma is firmer in texture than the lower. In Andropogoneae, lemmas are 2, the lower and upper, in this case, glumes are firmer and protective and thus lemmas are hyaline in texture. In some species of Setaria, Brachiaria and Panicum, Upper lemmas are transversely rugose. In most of the species of Andropogoneae and also in some genera like Arundinella, Tripogon, Chloris etc. upper lemmas possess 1 to 3 awns, which help in grains dispersal. In Jansenella griffithiana, upper lemma is deeply bifid with awn arising between the lobes. In Aristida, lemmas are linear-lanceolate and at apex it has 3 awns. (Plate 22)

            Paleas are scaly, borne on the floral axis directly facing the lemma, and flowers are in between them. In Andropogoneae group, paleas are hyaline in texture, whereas in Paniceae paleas are firmer.

Florets

            Florets comprise Lodicules, Androecium and Gynoecium. Lodicules are small scaly, oblong, cuneate-truncate or lanceolate, in shape, or lobed in various ways, usually 1–3 in numbers. In Aristida, lodicules are oblong-lanceolate, hyaline and 2 in number. In most of the Andropogoneae and Paniceae, lodicules are truncate and 2 in number. Flowers are attached to the base of the lodicules.  

            Androecium comprise 1–6 stamens, usually, most of the grasses have 3 stamens, but in Oryzeae they are 6. Each stamen has long, slender filament carrying two-celled anther. In some species of Pennisetum, anthers has hairs at the apex. Usually, anthers are yellow to deep purple in colour.           

            Gynoecium consists of ovary, ovules, style and stigmas. Ovaries are usually ovate–oblong with long style and plumose stigma to attract pollen from air.                               

Caryopsis

            Caryopsis is dry, one seeded, indehiscent fruit comprising fertilized ovary surrounded by the pericarp or remains of ovary wall.  On abaxial surface, at the base of the caryopsis there will be a depressed elliptic or ovate mark where the embryo is situated. On the other side, diametrically opposite is the hilum which represents the point of attachment of the ovule. Generally, shape and size of the caryopsis varies considerably usually, caryopses are ovate, obovate to lanceolate. In Dimeria and Aristida, caryopses are linear-lanceolate.  In genera like Brachiaria, Setaria and Sorghum, seeds are globular. Dispersal of the seeds is facilitated by appendages attached to it like awn of the lemma and bristle attached to spikelet. In most of the species belonging to the tribes, Andropogoneae and Arundineae, dispersal of the seed takes place with the aid of awns attached to the lemma, the seeds in this case remain inside the florets, awns often get stuck to body of animals thus functioning as dispersal agents. In Setaria and Pennisetum, spikelets have bristles at the base which work as a dispersal appendage. In Saccharum, spikelets are densely hairy and with wind these spikelets get drifted to long distances and disperse the seed. In Centotheca, the bristle, on lemma gets attached to the animal fur and helps in dispersal.