PHAXEROGAMIA. 



407 



In both Angiosperms and Gymnosperms, the pollen-tube is 

 formed by the outgrowth of the large vegetative cell : in both 

 cases the generative cell (after being set" free when necessary) 

 enters the pollen-tube together with the vegetative nucleus ; the 

 vegetative nucleus becomes disorganised, whilst the generative cell 

 undergoes division into two; either into two equal generative cells, 

 as is generally the case, or in two unequal cells only one of which 

 is generative (e.g. Taxus). More than one pollen-tube may be 

 developed from the microspore (Fig. 236). 



Thus the male individual in the Phanerogams is a prothallium 

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

 celled. 



The male cell is usually a small 

 nucleated primordial cell in the pol- 

 len-tube, and is either the original 

 generative cell itself, or a product 

 of its division ; but in some Gymno- 

 sperms (Cycas, Zamia, Ginkgo) it is 

 a large spirally-coiled inulticiliate 

 spermatozoid. It is eventually ex- 

 truded through the apex of the 

 pollen-tube. 



The Female Prothallium is de- 

 veloped in the interior of the macro- 

 spore (embryo- sac) in a similar 

 manner to that of the heterosporous 

 Pteridophyta : but, in the Phanero- 

 gams it does not at any period pro- 

 ject from the macrospore as it does 

 in the Pteridophyta, though this 

 occurs exceptionally in the Cyca- 

 daceae. 



The development of the prothal- 

 lium (or endosperm) is simple in the 

 Gymnosperms. The nucleus of the 

 macrospore divides ; repeated nuclear 

 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 multiply by division until the cavity of the niacro- 



Fio. 241. The female prothallinm 

 of Gymnosperms (t.g. Finns), shown 

 in a longitudinal section of the ovule 

 ( x about 15; diagrammatic) : 1 1 in- 

 tegument ; m micropyle ; K nacellus 

 (macrosporangium) ; E embr.vo-sac 

 (macrospore) ; t female prothallium 

 (endosperm), in which are situated, to- 

 wards the micropyle, two archegonia, 

 c, with neck h ; ps pollen-tube enter- 

 ing the neck of the left archegoniani ; 

 p pollen-grain seated on the apex of 

 the nucellns. 



