THE PLANT AND THE ANIMAL 



259 



filled with a darker green and thicker mass (fig. 75, I.). 

 If we observe such a swelling for some time (the obser- 

 vation must be made early in the morning, because in 

 the day-time this phenomenon ceases), we shall notice 

 that the green mass gathers into a round or rather an 

 oval lump, creeps out of the sack, which is ruptured 

 at the top, and begins to move (fig. 75, I.). This is a 

 large zoospore formed out of the protoplasm of our 

 water- weed. 



The movements exhibited by spore plants are not 

 limited to the zoospores. We saw in the previous 

 chapter that these plants are clearly differentiated in 

 sex, but for the sake of simplicity we chose cases 

 where both male and 

 female cells are non- 

 motile, and come into 

 contact only by fu- 

 sion. But in a greater 

 number of cases the 

 male cell is motile, 

 and therefore seeks out 

 the female which is 

 enclosed in a special 

 organ. In very rare 

 cases both male and 

 female cells are motile, 

 like the zoospores just 

 described : their move- j>j 

 ments bring them to- 

 gether, make them meet 

 and fuse into a single 

 mass, into one cell, 

 one spore. As a matter of fact in weeds, mosses, 

 ferns, horsetails, and clubmosses it is only the male 

 cell which is motile ; it also assumes most fre- 

 quently the shape of a rod twisted into a spiral and 

 provided with cilia. These so-called antherozoids are 



FIG. 75. 



