204 SEXUAL REPRODUCTION OF ECTOCARPUS 



form a gametospore. The nature and character of these bodies is 

 so imperfectly established that their behavior is entirely deter- 

 mined by external conditions. Low temperatures and bright 

 light tend to develop these bodies as zoospores, whereas, high 

 temperatures cause them to behave as gametes. (See Pandorina 

 and Hydrodictyon.) Other species of Ectocarpus show an ad- 

 vance over this stage. The gametes are alike, but it has been 

 observed that certain ones, the female gametes, have a shorter 

 period of motility and after coming to rest they attract the still 

 motile gametes, the males, and cause one to fuse with them 

 (Fig. 119, C, D). This is the simplest distinction that can be 

 pointed to as indicating a difference in the nature of the gametes 

 which we call sex. This variation in the period of motility of 

 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 Bctoearpus, 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 



