394 FOURTH GROUP.— SEED-PLANTS. 



after its growth has ceased. Portions of protoplasm, in the centre of each of which 

 there is usually a single nucleus ', become separated off from one another by walls 

 perpendicular to the wall of the embryo-sac, and are then invested on their inner side 

 also with walls of cellulose. In this way the embryo-sac is first lined with a single 

 layer of cells, and by several layers only at the parts indicated above. The cells of this 

 layer at once begin to multiply and thus ultimately fill the entire sac with endosperm, 

 provided the latter is not early displaced by the growing embryo. The cell-formation 

 round the free nuclei is not usually simultaneous throughout the whole embryo-sac, but 

 advances in it in a given direction ; it may have begun at the two extremities, while 

 free formation of nuclei is still going on in the central parts. If the embryo-sac 

 increases greatly in size, it may be a long time before it is filled with endosperm, as 

 in Ricinns. The centre of the sac in unripe seeds is filled with a clear vacuole-fluid ; 

 in the embryo-sac of the coco-nut which grows to an enormous size this fluid, the 

 milk of the coco-nut, is still found when the seed is fully ripe, and the tissue of the 

 endosperm forms a layer of only a few millimetres in thickness lining the inner side 

 of the seed coat. The formation of endosperm by free cell-formation, that is by 

 division of nuclei, at first without the formation of cells, these being only formed 

 subsequently, occurs, as has been already intimated, in plants whose embryo-sac 

 attains very large dimensions or grows very rapidly, while in narrow and slowly 

 growing sacs it is produced by the division of the sac itself into cells. In a few 

 families only, the formation of endosperm is rudimentary, being limited to the 

 temporary appearance of a few free nuclei or cells, as in Tropaeoliwi, Trapa, the 

 Naiadaceae, and Alismaceae ; even this rudimentary formation is wanting in the 

 Orchideae. 



During the formation of endosperm ihe embryo-sac usually increases in size, 

 and displaces whatever tissue of the nucellus there may still be surrounding it ; in a 

 few cases only is the nucellus wholly or partially preserved, and then it becomes 

 filled with food-material, like the endosperm, and takes its place as a receptacle of 

 reserve-material for the embryo; in the Scitamineae (Ca;/«rt) this i\ss\xe,t\ie pcn'sperm, 

 is largely developed, the endosperm being entirely absent ; in the Piperaceae and 

 Nymphaeaceae there is a small endosperm present in the ripe seed, but it lies in an 

 excavation in the much more copious perisperm. 



The seed-coat is meanwhile being developed from the integuments, and increases 

 with the increase in size of the embryo-sac and the endosperm which it contains. In 

 Cn'inan capense and in some other Amaryllideae, according to Hofmeister, the growing 

 endosperm bursts the seed-coat and even the wall of the ovary, its cells produce 

 chlorophyll, and its tissue continues succulent and forms intercellular spaces, which 

 does not happen elsewhere; in Ricinus a similar growth takes place in the germination 

 of the ripe seed in moist ground : by this means the seed-coat is ruptured and the 

 endosperm previously about eight to ten millimetres in length is hanged to a flat 

 broad sac some twenty or twenty-five millimetres in length, which encloses the 

 growing cotyledons till they have exhausted its food-material. 



' In some cases {^Corydalis cava, Staphylea pinnata, Armcria vulgaris and others) there are 

 several, two, three or four nuclei ; these then coalesce in a remarkable manner, as Strasburger has 

 shown, loc. cit., p. 26, and form a single nucleus. 



