628 PHANEROGAMS. 



the end of the floral axis itself (see pp. 492, 571); in Lemna and in sonne Aroideae 

 one or more ovules stand at the bottom of the cavity of the unilocular ovary. The 

 prevailing form of the ovule is anatropous ; but in Scitamineae, Gramineae, and some 

 other orders, campylotropous ovules occur ; in the Enantioblastae and a few Aroideae 

 they are orthotropous, either erect or pendulous. The nucellus is almost without 

 exception enclosed in two envelopes {Crinum however forms an exception). 



The Embryo-sac ^ generally remains surrounded by one layer of the tissue of the 

 nucellus till the time of fertilisation; the apex is sometimes destroyed so that the 

 embryo-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 top of 

 the embryo-sac (as in some Aroideae and Liliaceae). In Orchideae the growing 

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

 the apex of the nucellus; and this happens after fertilisation 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 odorans, 

 Ophrydeae). 



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

 sperm-cells in the parietal protoplasm follows quickly after fertilisation. They 

 soon unite into a layer of tissue and divide tangentially, 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, Gaged). 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. 586). 



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 Aroideae. But in all the other Monocotyledons which have no endosperm 

 (exalbuminous), Naiadeae, Potamogetoneae, Juncagineae, Alismaceae, Cannaceae, and 

 Orchideae, 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. 589) ; there are many points which are still 

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

 original small-celled mass of tissue of the embryo. 



^ See Hofmeister, Neue Beitrage (Abhandl. der konigl. Sachs. Gesellscli. der Wissensch. 

 vol, VII) ; also mpra, p. 576. 



