MONOCOrVLEDONS. crj 



The Embryo-Sac^ generally remain^ surrounded by one layer of the tissue of the 

 nucleus till the time of impregnation ; the apex is sometimes destroyed so that the 

 embr\-o-sac projects (as in Hemerocallis, Crocus, Gladiolus, &c.) ; but, on the other 

 hand, the apex not unfrequently remains as a cap of tissue covering the apex of 

 the embryo-sac (as in some Aroideaj and Lihacese). In Orchidese the growing- 

 embryo-sac completely destroys the layer of tissue that envelopes it together with 

 the apex of the nucleus; and this happens after impregnation in all the other 

 Monocotyledons that possess an endosperm, and in this case the embryo- sac some- 

 times advances even to the inner integument and destroys it {Allium odor am, 

 Ophrydeae). 



In the greater number of IMonocotyledons a copious development of endo- 

 sperm-cells follows quickly after impregnation ; these are all formed simultaneously, 

 and remain free in the parietal protoplasm. When they lie near together they soon 

 unite into a layer of tissue and divide tangentially, new cells being formed at the 

 same time by free cell-formation on the inner side of the first layer which behave in 

 the same manner, until at length the embryo- sac is filled with radial rows of cells 

 the result of division. Narrow embryo-sacs are filled up by the growth of the 

 first endosperm-cells; but sometimes the cells formed by free cell-formation in 

 the parietal layer of protoplasm constitute at first a loose mass which fills up the 

 embryo-sac and only closes up into a tissue at a later period {e.g. Leucojum, 

 Gagea). The narrow embryo-sac of Pistia is filled up by a row of broad disc- 

 shaped cells which lie in it like transverse compartments and are perhaps the result 

 of division of the sac itself In some Aroideae only a part of the embryo-sac is 

 filled with endosperm, the rest remaining empty. 



The endosperm still continues to grow after it has filled up the embryo-sac, the 

 seed which it fills increasing also in size. It has already been mentioned how 

 considerable this growth is in Crinum (p. 512). 



In all those Monocotyledons which form an endosperm (albuminous), it becomes 

 closed up into a continuous tissue enveloping the embryo before this has completed 

 its growth. By the growth of the embryo a part of the endosperm which surrounds 

 it is again forced aside ; and on this displacement depends the lateral position of 

 the embryo in Grasses by the side of the endosperm, and the absence of this latter 

 in some Aroidese. But in all the other Monocotyledons which have no endosperm 

 (exalbuminous), Naiadese, Potamogetonese, Juncaginex, Alismaceae, Cannaceae, and 

 Orchidese, its formation is altogether suppressed, or transitory preparations for it 

 only take place. 



On the first origin of the embryo reference must be made to what was said in 

 the Introduction to Angiosperms (p. 510) ; there are many points which are still 

 doubtful in the formation of the plumule, scutellum (in Grasses), and root, from the 

 orio^inal small-celled mass of tissue of the embryo. 



^ See Ilofmeister, Neue Beitrage (Abhandl. der konigl. Sllchs. GeselLsch. der Wissensch. 

 vol. VII). 



