DEVELOPMENT OF PLANTS 193 



the gametes must be due to an essential difference in the material 

 or substance of which they are composed, although there is no 

 external evidence of this. Certain species of this same genus 

 also reveal a variation in the size of the cells from which the 

 gametes are derived. In these species, the sex is clearly indi- 

 cated by the larger size of the female cell and it is noticeable 

 that there is a decided tendency for the smaller gametes to fuse 

 with the larger and more slowly moving females. It is also 

 worthy of note as these gametes become differentiated and their 

 sex more evident that they are less able to behave as zoospores 

 and grow directly into new plants. When the gametes produced 

 by the plant are all alike they may readily be made to grow into 

 new plants, but this is rarely the case where they can be dis- 

 tinguished as male and female. Thus, in Ectocarpus, we have 

 a most remarkable series of variations that indicate how sexuality 

 has arisen from the asexual condition and also how sex finally 

 became characterized by a shorter motile period in the female 

 gamete. The character of the female finally became more pro- 

 nounced, owing to its better nourishment and consequent increase 

 in size and slower movements. A more perfect illustration of 

 the evolution of sex is not found in nature. 



The differentiation of reproductive parts, as will be frequently 

 noted, does not keep pace necessarily with the evolution of the 

 plant body. It is evident that sexuality has arisen independently 

 at different levels in the plant world and in groups in no wise 

 connected. We have noted the same state of sexuality in some 

 of the motile green algae, i. e., in Hydrodictyon and Ulothrix, 

 and now again in Ectocarpus although these plants are widely 

 separated as far as relationship is concerned and exhibit a marked 

 variation in the development of the plant body. It is also fre- 

 quently to be noted that plants may get along very well indeed 

 with only a sexual or an asexual method of reproduction. 



(b) The Coarser Brown Algae, the Kelps. — This latter condi- 

 tion is well illustrated in the kelps, plants related to Ectocarpus, 

 which include the largest and most highly organized forms of 

 all the algae. Indeed some of these forms are quite comparable 

 in size with our shrubs and trees. The Laminarias (Fig. 120, A) 

 14 



