A NGIOSPERMS. MONOCOTYLEDONS. 441 



or more ovules are placed at the bottom of the cavity of the unilocular ovary of some 

 Aroideae and of Lemna. The prevailing form of the ovule is anatropous ; campylo- 

 tropous ovules occur in the Scitamineae, Gramineae and some other cases ; ortho- 

 tropous, both erect and pendulous, are found in the Enantioblastae and certain of the 

 Aroideae. The nucellus is almost invariably surrounded with two envelopes ; 

 Crinum is an exception. 



The embryo-sac^ usually continues to be covered by a layer of the tissue of the 

 nucellus up to the time of fertilisation ; in some cases the apex of the nucellus is 

 destroyed and the embryo-sac protrudes, as in Hemerocallis, Crocus, Gladiolus, etc., 

 but it often remains intact as a cap of tissue covering the apex of the embryo-sac, -as 

 in some Aroideae and Liliaceae; in Orchids the growing embryo-sac completely 

 destroys the layer of tissue that surrounds it together with the apex of the nucellus ; 

 the same thing happens after fertilisation in all other Monocotyledons that form an 

 endosperm, and sometimes the embryo-sac even attacks and destroys the inner 

 integument, as in Allium odorans and the Ophrydeae. 



In the majority of Monocotyledons fertilisation is soon followed by a copious 

 development of endosperm-cells by free cell-formation ; the nuclei formed by division 

 first of the nucleus of the embryo-sac and then of its daughter-cells are imbedded 

 in the layer of protoplasm which lines the wall of the sac, and become centres of 

 cell-formation. Narrow embryo-sacs are filled by the growth of the free endosperm 

 cells first formed ; in some cases the free cells formed in the parietal protoplasm 

 float loose in it at first, and afterwards unite into a tissue as in Leucojum and Gagea ; 

 the narrow embryo-sac of Pistia is filled with a row of broad discoid cells, which lie 

 in it like transverse compartments and may perhaps be produced by the division of 

 the sac itself, In some Aroideae one part only of the embryo-sac is filled with en- 

 dosperm, while the rest has none. The endosperm continues to grow after it has 

 filled the sac, while the seed which it fills also increases in size ; it was mentioned 

 above (p. 394) how considerable this growth is in Crinum. 



In all Monocotyledons which form an endosperm, the endosperm ultimately 

 forms a continuous tissue enveloping the embryo before the latter has ceased to grow ; 

 the growth therefore of the embryo displaces a part of the endosperm that surrounds 

 it, and it is this displacement which causes the lateral position of the embryo of the 

 Gramineae by the side of the endosperm, and the absence of endosperm in some 

 Aroideae. In Monocotyledons in which the mature seed has no endosperm 

 (exalbuminous seed), the Naiadaceae, Hydrocharideae, Juncagineae, Alismaceae, 

 Canneae, and Orchideae, no endosperm is formed at all, or only transitory prepara- 

 tions for its formation are observed. 



The first formation of the embryo has been described in the Introduction to the 

 Angiosperms. 



In respect of their histology 2 Monocotyledons differ from Dicotyledons and Gymno- 

 sperms chiefly in the course of the vascular bundles in the stem and in the absence of a 

 true cambium-layer. The transverse section of the stem of most Monocotyledons does 



1 Hofmeister, Neue Beitr. (Abh. d. K. Sachs. Ges. d. Wiss. VII) ; see also the works cited 

 above, pages 382 and 390. 



2 See De Bary, Vgl. Anat. p. 262 of the English Edition, and the literature cited in that work. 



