461 BOTANY OF THE LIVING PLANT 



If two organisms, each consisting of only a single cell, fuse 

 to form one, the immediate result is a diminution in number 

 to one half. This' occurs in various instances, and Spirogyra is a 

 case in point, for each cell of its filament is properly recognised 

 as an individual. ' The same occurs in various other unicellular 

 Animals and Plants. As these are probably primitive, they suggest 

 that in the first instance syngamy -was not a means of increase in 

 number of individuals, though in all the higher Plants and Animals 

 this appears to be its natural consequence. Some believe that the 

 chief advantage following on sexual fusion in such simple organisms 

 lay in nutrition. If during repeated fissions cell-division was more 

 rapid than nutritive recuperation, then fusion of two cells would be a 

 possible form of recovery. But such fusion appears to bring with it 

 also a stimulus to fresh activity of growth and division, which may 

 break out at once, though in primitive organisms it follows usually after 

 a period of rest. With this fusion there follows also the pooling of such 

 qualities as the fusing cells themselves possess. So far as these 

 qualities can be transmitted to the offspring, the mechanism of fusion 

 offers the opportunity for their transmission ; and it is significant 

 that the fusing gametes are as a rule distinct in origin. For instance, 

 the pairing gametes of Ulothrix (p. 392), or of Ectocarpus (p. 383), 

 originate from different gametangia ; and the distinctness of origin 

 is still more marked in many plants higher in the scale. It seems 

 probable that such advantages as these, viz. nutritive recovery, 

 stimulus to further development, and hereditary transmission, have 

 favoured a constant recurrence of syngamy. In the long run heredi- 

 tary transmission has been the most important. 



DIFFERENTIATION OF SEX. 



Fusion of isogametes once established led to sexual differentiation 

 in many distinct phyletic lines, both of Animals and Plants. Com- 

 parison of closely related forms is the basis of this conclusion ; and 

 a particularly convincing example is seen in the Brown Algae (pp. 380- 

 385). The distinction is there found to be first a difference in 

 behaviour rather than of form (Ectocarpus siliculosus). Next, a differ- 

 ence in size as well as in behaviour marks the female as distinct from 

 the male, as in E. secundus. In Cutleria that difference is still more 

 accentuated, and the larger female gamete soon loses its motility. In 

 Fucus the difference in size is very great indeed, and the large female 

 egg is never motile at all. Various other phyletic lines could be 



