y:/. PHANEROGAMS. 



The Embryo-sac'^ is formed by the early enlargement of a cell lying nearly 

 in the centre of the young nucleus, the surrounding tissue remaining small-celled 

 and for some time afterwards in the condition of primary meristem, the growth of 

 the entire ovule being continued in this manner. In Orchidese (Fig. 366, p. 502), 

 where its structure is very simple, the young ovule consists of a single layer 

 of cells surrounding an axial row; the uppermost of these latter cells becomes 

 the embryo-sac, and begins to enlarge even before the integuments are developed 

 from the peripheral layer. Hofmeister is inclined to apply this description to all 

 ovules, and to consider the embryo-sac as being always developed from a cell be- 

 longing to an axial row which passes through the ovule. It is, however, very 

 difficult to prove the existence of such an axial row of cells in very small-celled 

 ovules, especially those of Dicotyledons ; and even among INIonocotyledons the 

 description of the ovules of Orchidea^ does not seem always to apply, as may be 

 seen (Fig. 367, p. 503) in the case of Funkia. It also sometimes happens among 

 Angiosperms — reminding one of the phenomena in Taxus among Gymnosperms — 

 that several embryo-sacs are at first formed ; this is the case, for instance, accord- 

 ing to Tulasne, in Cruciferoe, where however only one attains full development. 

 The multiplicity of embryo-sacs in the ovary of Viscum cannot at present be 

 included under this head, since the absence of any differentiation of the ovule 

 makes it uncertain whether the mass of tissue in the ovary to which we have already 

 alluded must be regarded as the equivalent of one or of several ovules. 



The further behaviour of the embryo-sac of Angiosperms differs in many ways 

 from that of Gymnosperms. In Gymnosperms it remains surrounded by a thick 

 layer of the tissue of the nucleus till after fertilisation has taken place ; it is com- 

 paratively small, and is surmounted by a strongly developed nuclear protuberance. In 

 Angiosperms, on the other hand, the embryo-sac has grown considerably even before 

 fertilisation ; it usually supplants the surrounding tissue of the nucleus so far that it 

 remains enveloped by only a thin layer of it, or is even in actual contact with the 

 inner surface of the inner integument, as in Orchideae (Fig. 366, VII). In such 

 cases the tissue of the apex of the ovule often still remains entire (as in Aroidece), 

 but frequently the apex of the embryo-sac bursts through it, and projects into the 

 micropyle (as in Crocus and Labiatce), or even grows out beyond it as a long tube 

 {e.g. Santalum). The middle and lower part of the sac also frequently extends con- 

 siderably; in many gamopetalous Dicotyledons it puts out vermiform appendages 

 which penetrate into and destroy the tissue of the integument, as in Rhinanthus, 

 Lathraea, and some Labiatse. While this process of growth is proceeding, the proto- 

 plasm which at first fills up the whole sac becomes full of vacuoles ; a large sap- 

 cavity arises surrounded by a parietal mass of protoplasm, which accumulates 

 especially in the apical prominence and at the bottom of the embryo-sac, while 

 threads, in which currents are visible, radiate to the walls from the protoplasm 

 which envelopes the nucleus. 



After this state of things has arisen, but still long before fertilisation, and even 

 before the development of the embryonic vesicles, one or more cells are formed in 



* The following is mostly founded on Hofmeister's Neue Beitr.'ige (Abh. der kon. s"ichs. Ges. 

 der Wiss. VI and VII). 



