[Illustration: 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.] 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.” [Illustration: Fig. 38. Longitudinal median section of the caryopsis of a grass--e.g. _Lolium_ (× about 35). _p_, pericarp; _t_, attachment to axis; _m_, position of micropyle; _E_, endosperm; _A_, its aleurone layer; _l_, folded leaves of plumule; _r_, radicle; _r´´_, secondary roots; _RC_, root-cap; _s_, scutellum. The dark line _e_ represents the surface where the face of the scutellum is applied to the endosperm and where absorption of the latter takes place.] 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. [Illustration: Fig. 39. Transverse section (highly magnified) through a grain of _Brachypodium pinnatum_ taken about half-way up. 1, epidermis; 2, pericarp; 3, remains of the true seed-coat; 4, vascular bundle; 5, remains of nucellus; _P_, epidermis of nucellus; _G_, aleurone layer: remarkable in being several cells thick; _E_, endosperm. Harz.] Thirdly, the outermost layer or layers of cells of the endosperm are filled with proteids, and are known as the Aleurone layer. (Fig. 38, _A_.)