3i8 THE BIOLOGY OF FLOWERING PLANTS 



§ 2. Sexual and Asexual Reproduction 



We may now put a more fundamental question. The 

 mosses, the ferns and their allies, the gymnosperms, and 

 the angiosperms all show normally an alternation of genera- 

 tions ; they have two distinct types of reproduction. What 

 different functions do these serve ? Why should not sexual 

 reproduction alone suffice, or asexual reproduction alone, 

 for by either a multitude of new individuals may arise ? 

 To answer this completely is not possible, for the essential 

 nature of sexual reproduction is not fully understood. 



If we try to trace the stages in the evolution of sexual 

 reproduction in the plant kingdom we find that we must 

 go back to the algae, for in the land flora the gametes are 

 throughout highly specialised. In the genus Chlamydo- 

 monas, unicellular, free-swimming, green algas, reproduction 

 occurs by internal division into a number (2 to 8) of 

 zoospores, each of which is just like the mother cell. When 

 these spores escape by the rupture of the mother cell wall 

 they increase in size, secrete a cell wall, and so reach the 

 adult state in a very simple fashion. The mature cell may 

 also give rise to gametes. 



These may be indistinguishable morphologically from 

 the zoospores, or they may be smaller, with slight structural 

 differences, as many as sixty-four being formed in a single 

 cell. On escaping, the gametes swim about freely but 

 do not grow into mature individuals. They fuse in pairs, 

 form a resting zygote, with a thick wall and reserve food. 

 The zygote germinates by dividing internally into four 

 zoospores. Here we have typical sexual reproduction in 

 which the reproductive cell proceeds to develop only after 

 union with, or fertilisation by, another reproductive cell. 

 It is on a low level because there is no difference between 

 the two gametes, and little difference between the gamete, 

 the zoospore, and the vegetative cell. Yet this simple 

 condition does not obtain in all the species of the genus. 



In Chlamydomonas grandis the two gametes are equal 

 and naked ; they come together point to point and fuse 



