1 68 Elementary Biology. 



the two upper cells form the sole remnant left of the canal 

 walls of the primitive ovarium. The three cells formed at 

 the opposite pole may similarly be looked on as the rudi- 

 ment of the thallus, probably that portion of it which would 

 give rise, if completely developed, to spermaria, just as the 

 vegetative cell of the spermospore, no doubt, contains 

 chemical compounds necessary to the development of an 

 ovarium. 



Fertilisation. - In the life-history of vascular plants we 

 have hitherto found that the male elements have been 

 motile cells termed sperms, which in virtue of their move- 

 ment have been able to migrate from the thallus bearing 

 them to the female organs carried in other thalli. The 

 pollen gram of a lily is itself, however, a non-motile struc- 

 ture, and no sperms are formed. How, then, is the male 

 protoplasm to be carried to the ovum, and what in this case 

 takes the place of the sperm in fertilisation ? 



Notwithstanding the fact that the majority of flowers are 

 hermaphrodite, i.e. bear both male and female elements, 

 it has been experimentally proved that, if an ovum be fer- 

 tilised by the pollen of the same flower, the result, if 

 there be any, is a weak and diminutive embryo, often in- 

 capable of developing into a perfect plant ; whilst, on the 

 other hand, if the ovum be fertilised by pollen taken from 

 another flower, a healthy and vigorous embryo is produced. 

 Thus cross-fertilisation tends to the maintenance of healthy 

 tribal life, whilst self-fertilisation tends to produce a dwarf 

 and weakly progeny, and ultimately extinction of the race 

 unless cross-fertilisation intervenes and saves it. This law 

 has a number of exceptions, but holds good in the great 

 majority of cases. Granted, then, that foreign pollen (of 

 course from a plant of the same kind) must be employed in 

 fertilisation, how is the pollen to be conveyed from the 

 male sporophyll of one flower to the female sporophyll of 

 another, since the pollen grains, or spermospores, are them- 

 selves non-motile, and do not produce any motile sperms ? 



