132 THE PLANT CELL. 



i. All outer wall, which soon becomes thickened, the extine, from 

 which are produced two lateral lobes ; these possess reticulations, and 

 are useful in buoying up the microspores during dispersion by the wind 



ii. An inner and thinner wall, the inline. 



iii. Internally, cytoplasm and a large nucleus. 



Maturation of the microspore takes place either in the pollen- 

 sac or whilst it is lying upon the apex of the nucellus in the ovule, 

 and consists in the cutting off of certain cells from the main mass ; 

 thus, the first cell to arise is the protballial cell, which may divide 

 again. The second division, which cuts off a cell from the larger 

 remaining cell, gives rise to the antheridial or generative cell ; 

 whilst the large cell now left is the vegetative cell, from which 

 the pollen-tube is produced. Later on the generative cell divides 

 into a stalk-cell and the generative cell proper (see Fig. 98), this 

 usually occurring after the pollen-tube has been formed. The 

 later changes are best described under fertilisation. It is note- 

 worthy that in Ginkgo and the Cycadece the generative cells 

 are further differentiated into antherozooids (ciliated motile cells) 

 of a peculiar type.* This process thus links these groups with 

 the Pteridophyta (Heterosporous type). 



6. The Macrospore ; its Origin and Maturation The embryo- 

 sac (macrospore) has an origin similar to that of the Angio- 

 sperms. An archesporial cell arises just beneath the epidermis 

 (or oftener rather deeper) of the nucellus of the ovule 

 (macrosporangium), this latter being situated upon the upper 

 surface of a carpellary leaf (macrosporophyll) of the female 

 cone (see Fig. 101). A primary tapetal cell is cut off from 

 the apical end of the archesporial cell, and also cap-cells from 

 the lower larger cell. The remaining large cell is the embryo- 

 sac (macrospore), which soon enlarges to many times its 

 original size. The next change which occurs is the division 

 of the cytoplasm and nucleus of this macrospore into a number 

 of free cells which soon develop cell-walls and undergo further 

 division, and the tissue which is ultimately formed by this 

 process fills the embryo-sac, and is known as a prothallium 

 (incorrectly termed endosperm). 



At the upper (micropylar) end of this prothallium now arise 



_ * For a very good account of the formation of the antherozooids of 

 Ginkgo, see Tlie Journal of Applied Microscopy and Lahoratorv Methods 

 for May, 1902. " 



