Ch. VI, 7] 



SIGNIFICANCE OF SEX 



303 



alike and contributing the same in kind to the offspring. 

 Such is the conclusion indicated by modern research. If, 

 now, the two sexes thus contribute equally to the constitu- 

 tion of the offspring, where lies the essential difference be- 

 tween them, or, more exactly, why does sex exist at all ? 



For light upon this question we turn to the sexual methods 

 and structures presented by the different groups of existent 

 plants, from the lowest to the highest. These will be found 

 described in full in Part II of this book, but for our present 

 purposes may be sum- 

 marized as follows. 



I. There are plants of 

 simple kinds, notably 

 Bacteria and some low 

 Alga?, which reproduce 

 solely by division, that is, 

 the single cells constitut- 

 ing the adults simply di- 

 vide across and grow to 

 full size (Fig. 210), pre- 

 cisely as do meristematic 

 cells already described in 

 the higher plants (page 



299). Here is represented a stage of reproduction in which 

 there is neither fertilization nor sex. 



II. There are several known Algae, of grade somewhat 

 higher than those just mentioned, in which the plants are 

 all alike, and produce small reproductive cells called gametes, 

 likewise all alike, and provided with swimming appendages. 

 These gametes are thrown out into the water, where, swim- 

 ming freely about, they come together at haphazard and 

 fuse, uniting their nuclei, quite in the manner of the fer- 

 tilization of the higher plants (Fig. 215) ; and from the re- 

 sult ing cell a new plant develops. Here is evidently repre- 

 sented a stage in which fertilisation occurs, but without 

 any difference between the sexes. 



Fig. 215.- 

 Protosiphon, 

 magnified, 

 "zygote." 



- Gametes of the simple Alga 

 in process of fusion ; highly 

 On the right a complete 



