558 BOTANY OF THE LIVING PLANT 



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half. This occurs in various instances, and Spirogyra is a case in 

 point, for each cell of its filament is properly recognised as an indivi- 

 dual. 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 w T as more rapid than nutritive recupera- 

 tion, then fusion of two cells would be a possible form of recovery. 

 But such fusio # n 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 

 it (see later) ; and it is significant that the fusing gametes are as 

 a rule distinct in origin. For instance, the pairing gametes of Ulothrix 

 (p. 365), or of Ectocarpus (p. 380), originate from different gamet- 

 angia ; 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 hereditary transmission has been the 

 most important. 



Differentiation of Gametes. 



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. 

 380-382). 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 



