248 



AND PHANEROCAMS. 



is now formed, and it consists in its most complete development, 

 as in this instance, of three parts : 



(1) The testa of the seed, formed by the enveloping integuments, 

 with the remainder of the tissue of the nucellus lying outside the 

 embryo-sac (the macrosporangium). 



(2) The endosperm or prothallium. 



(3) The embryo. 



The reduction in the ANGIOSPERMS is carried to the extreme 

 limit. In the embryo-sac (the macrospore) the nucleus by con- 

 tinued division produces a prothallium consisting of primordial 

 cells (Fig. 254). In the upper end of the embryo-sac (which is 

 nearest the micropyle) are three cells, two of which are termed 

 the "co-operating cells" (synergidcv) and the third is the oosphere. 

 Three others are placed at the opposite end of the embryo-sac and 

 are therefore termed the ' antipodal cells." Finally, a large cell is 

 also formed, which occupies the space between the two groups and 

 whose cell-nucleus, the central definitive nucleus, lies in the centre 

 of the embryo-sac. These primordial cells are the slight rem- 

 nant of the prothallium. The entire structure of the archegonium, 

 with its neck and canal-cells, has disappeared, and nothing is left 

 but the indispensable oosphere. When the oosphere has been 



fertilised, and has com- 

 J I menced the cellular divi- 

 sions which lead to the 

 formation of the embryo 

 (Fig. 255), the synergida> 

 and antipodal cells are 

 absorbed, and a cell-for- 

 mation begins by a new 

 process which emanates 

 from the definitive nu- 

 cleus and by which a pa- 

 renchymatous cell-tissue, 

 the nutritive-tissue, arises 

 which may perhaps be 

 considered as homologous 

 Avith the endosperm of the 

 Gymnosperms. The dif- 

 ference is that the nutritive-tissue of the Angiosperms is formed 

 in two parts with an intervening interruption ; the primary 

 nutritive-tissue is first formed, and after fertilisation is absorbed, 



r 



Fie. 255. Diagrammatic longitudinal section 

 through an anatropous ovule shortly after fertili- 

 sation ; a. and i are the two integuments ; / the 

 funicle ; ): the nncellus; S the embryo-sac, with the 

 incipient formation of nutritive-tissue ; E the 

 embryo; P the pollen-tube passing through the 

 micropyle () to the oosphere. 



