Fig. 37. Pollen-grains adherent to the papillæ of the stigma, on which they germinate, sending the pollen-tubes down between the cells. Kerner. × about 100. are ready (protandrous). Among exotic grasses, many are diœcious or monœcious—i.e. the flowers contain stamens only or ovary only, on each plant, or on different inflorescences of the same plant respectively—and even in our own Holcus and Arrhenatherum this state of affairs is partially represented, since one flower of the spikelet is male only.
In some grasses, e.g. Rye, however, it appears improbable that cross-fertilisation ever occurs, since the paleæ do not open, and the pollen falls on to the stigma direct; and in Leersia and the foreign Amphicarpum the spikelets are completely cleistogamous, those of the latter being on stalks close to the ground which push the flowers into the soil, where pollination and fructification are accomplished.
Hybrid grasses are by no means uncommon. To say nothing of the numerous cross-bred Wheats and Barleys, artificial hybrids have been raised between Wheat and Rye. In the Maize an astonishing number of selected cross-breeds have been obtained, and, among
When the pollen-grain, having adhered to the hairy stigma, has begun to germinate, the resulting pollen-tube creeps down between the cells of the stigma, and hands over its enclosed nuclei to the embryo-sac, where fertilisation of the egg-cell is accomplished, by the fusion of one of the pollen nuclei with the nucleus of the egg-cell. As the resulting embryo developes, the sac becomes filled with endosperm-cells charged with starch-grains or sugar, and in the ripe seed the embryo is always found affixed laterally and below to this endosperm—a point of distinction from Sedges, where the embryo is buried in the endosperm.
The ripe seed fills the ovary, and its outer walls usually fuse with those of the carpel, forming the well-known Caryopsis or “grain."
If such a “grain" is carefully examined, three chief parts are visible in addition to the embryo. (Fig. [38].) Firstly, we find on the outside the fused seed-and fruitcoats, differing in the number of layers and in the microscopic characters of the cells, some of which characters can be employed in diagnoses. (Fig. [39].)
Secondly, the great mass of the “grain" internally is composed of delicate cells filled in most cases with starch-grains, the sizes, shapes and arrangement of which can also be employed for diagnoses—e.g. the compound grains of the Aveneæ and Festuceæ are different from the simple polyhedral or rounded grains of the Andropogoneæ and Maydeæ, and some races of Maize have sugar and soluble starch instead of grains of the latter.
