1^6 Plants and their Ways in South Africa 



In Gymnosperms, of which Pimn may be taken as a type, 

 the nucleus of the megaspore divides to form a mass of cells, 

 the prothallus or endosperm. This forms several (three or four 



commonly in Pinus pinea) arche- 

 gonia at the end toward the 

 micropyle. 



In Angiosperms the nucleus 

 of the megaspore typically divides 

 into two, which pass to opposite 

 ends of the spore. These each 

 divide twice, forming four nuclei 

 at each end. Of these, three at 

 the micropylar end form the egg 

 apparatus, the largest of the three 

 becoming the egg. Of the four 

 at the opposite end, three surround 

 themselves with walls. They are 

 regarded by some as constituting 

 a rudimentary prothallus and by 

 others as a second egg apparatus 

 which no longer functions as such. 

 One of the four nuclei at each 

 end passes to the centre of the 

 megaspore. 'I'hese fuse and after 

 fertilization, this, the second- 

 ary nucleus develops the endo- 

 sperm. 



It is an advantage for the 

 megaspore to remain enclosed by 

 the food-storing cells of the nucel- 

 lus, but a serious problem now 

 presents itself ; How is the sperm- 

 cell to reach the egg? In the 

 lower types they did so by swim- 

 ming. But obviously swimming wimld a\'ail liltle under present 

 conditions, and it has become practically a losl .ucomiilishment 

 of the sperm-cells of Spcrmaphytes. The difficulty has been 

 surmounted by the microspores (pollen grains). These have 



Fig. 2ro. — Diagrammatic view of 

 ovary of a Dicotyledon contain- 

 ing one erect, basal, anatropous 

 ovule. Tne pollen tube has 

 discharged its two nuclei, n, 

 nucellus ; em, embryo sac; a, 

 antipodal cells ; h, two polar 

 nuclei which unite to form the 

 central or definitive nucleus ; f, 

 egg apparatus with sperm cell 

 applied to the egg cell, d ; s, 

 seed coat ; r, nuclei discharged 

 by the pollen tube ; /, funicle ; 

 /, pollen tube which lias entered 

 tlirotigh the micropyle. 



