DEVELOPMENT OF PLANTS 183 



plant. The tissues are becoming more specialized and the work 

 performed by the plant is apportioned to special cells and organs. 

 The zoospores have essentially the same structure and mode of 

 growing into new plants as noted in Ulothrix. The cilia, how- 

 ever, are laterally attached and this is a characteristic feature 

 of the zoospores of the Phaeophyceae generally (Fig. 119, 

 A, g). 



The sexual reproduction of Ectocarpus exhibits some of the 

 most instructive variations to be found in all the algae. The 

 gametes are formed in gametangia that become divided into 

 great numbers of very small cubical cells, each one of which 

 produces a single gamete (Fig. 119, B). Thus the origin of 

 the gametes is more specialized than in the case of the green 

 algae where numerous gametes were formed in a single cell. 

 The gametes themselves, however, show the same range of vari- 

 ations as previously noted. In the simpler types of this genus 

 they are quite alike and on the same plane of differentiation as 

 in Ulothrix, since they sometimes develop as zoospores, directly 

 forming the plant, or they may behave as gemetes and fuse to 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 Hydrodyction.) 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- 



