1 1 o Practical Plant Biology. 



of its sexual cells. And in the lower forms of vegetable life, such 

 as we have been considering hitherto, it is usual to find that 

 differentiation of the somatic cells is associated with differentiation 

 of the reproductive also. Hence we are led to suppose that the 

 sexual cells of the simplest and therefore the most primitive 

 organisms were similar to one another. Thus while the fusing 

 gametes of Chlamydomonas are identical with one another, a 

 physiological distinction is already visible in such a simple 

 organism as Spirogyra, and in the more complex organisms, 

 Vaucheria and Volvox, where already some somatic differentiation 

 is perceptible, marked morphological and structural differentiation 

 is apparent in the gametes. They are plainly distinguishable as 

 the quiescent large ovum and the motile minute sperm. This 

 differentiation is still more marked in Fucus. 



Following a somewhat similar line of thought and finding 

 that sexual reprcduction normally occurs periodically in the life 

 history of the more complex forms, while in the simpler or lower 

 forms reproduction is either always, or more frequently, effected 

 by asexual cells (spores), we surmise that asexual, reprcduction is 

 the more primitive method. Asexual reproduction may be 

 effected by simple cell division or budding (Spirogyra, bacteria, 

 yeast), or by segmentation of the protoplasm within the cell-wall into 

 two or more masses (bacteria, yeast, Chlamydomonas). The spores 

 when liberated are smaller than the adult cell and evidently, unless 

 they find adequate supplies to enable them to meet the necessary 

 demands of energy and material, they will perish. Thus we often 

 find spores specially provided with a store of food material such 

 as fat, carbohydrates and proteins. Should they exhaust or lack 

 this they will be exposed to the hunger stimulus. We may 

 imagine that those which are naked, or only incompletely covered 

 with a cell-wall, will, in response to .this stimulus, respond as the 

 simplest organisms do, by ingesting or engulfing other cells. The 

 spores being set free usually in numbers together will naturally 

 under starvation conditions tend to ingest one another and thus 

 obtain sufficient nutriment to persist and carry on till change of 

 conditions provides for their development. This procedure, 

 which in the first instance was supposedly initiated by chance, 

 might become normal by the recurrence of the same conditions. 

 In some process like this we may present to ourselves the origin 

 of sexual reproduction. Support of this view is found in the 

 observation that the sexual cells of many of the lower plants may 

 be made to act as spores (i.e. reproduce the plant without fertilisa- 

 tion) if they are richly supplied with food, The germination of 



