450 



PART III. — THE €LASSIFICATION OF PLANTS. 



consisting of but few cells, and the antheridium is at most two- 

 celled : the generative cell represents a spermatozoid-mother-cell, 

 the protoplasm of which is not, however, differentiated into a 

 spermatozoid, but simply constitutes a male cell. 



The male cell is a small nucleated primordial cell in the pollen- 

 tube, and is either the original generative cell itself, or a product 

 of its division. It is eventually extruded through the apex of 

 the pollen-tube. 



The Female Prothallium is developed in the interior of the 

 macrospore (embryo-sac) in a similar manner to that of the 

 heterosporous Pteridophyta : but, in the Phanerogams it does not 

 at any period project from the macrospore as it does in the 

 Pteridophyta, though this occurs exceptionally in the Cycadaceae 

 among Grymnosperms, and in Avicennia among Angiosperms. 



The development of the prothal- 

 lium (or endosperm) is simple in the 

 Gymnosperms. The nucleus of the 

 macrospore divides ; repeated nu- 

 clear division takes place, until a 

 large number of nuclei are formed 

 which lie in the protoplasm round 

 the wall of the macrospore ; between 

 these nuclei cell-walls are developed, 

 so that a cellular tissue is produced, 

 the cells of which grow and multi- 

 ply by division until the cavity of 

 the macrospore is entirely filled with 

 this tissue which constitutes the 

 prothallium. In Gnetum, however, 

 the development of the prothallium 

 is not completed until fertilisation 

 has taken place. 



In the Angiosperms the develop- 

 ment of the prothallium is more 

 complicated in that it generally 

 takes place in two stages, the one 

 preceding, the other following, fer- 

 tilisation. The nucleus of the ma- 

 crospore divides into two : of these 

 the one travels to the micropylar pole, the other to the chalazal 

 pole, of the macrospore ; each nucleus then divides, and each of 



Fig. 291.— The female prothallium 

 of Gymnosperms, shown in a longitu- 

 dinal section of the ovule ( x about 

 15; diagrammatic): u integument; 

 m micropyle. K Nucellus (macrospo- 

 rangium). E Embryo-sac (macro- 

 spore) ; e female prothallium (endo- 

 sperm), in which are situated, towards 

 the micropyle, two archegonia, c, 

 with neck ?i; ps pollen-tube entering 

 the neck of tie left archegonium; 

 p pollen-grain seated on the apex of 

 the nucellus. 



