422 VEGETABLE PHYSIOLOGY 



produced in larger numbers and have only two cilia each 

 (fig. 169). After they are set free into the water they 

 swim about for some time, and then they usually fuse 

 together in pairs, nucleus joining nucleus and protoplasm 

 uniting with protoplasm. The new body so formed is 

 known as a zygospore. After a period of rest it can give 

 rise to a new filament. As there is no difference between 

 the cells which unite to form this structure, they are 

 frequently called planogametes. 



In the Zygnemece and the Mesocarpece the gametes are 

 solitary and non-motile and do not escape from the cells 

 in which they are formed. Two filaments take part in 

 the fusion of the gametes ; these are found lying close 

 together in the water ; from a cell of each filament a 

 protrusion grows out towards the other and the two come 

 into contact and join, the separating walls breaking down. 

 The contents of one cell pass over into the other through 

 the channel so formed, or the contents of both the cells 

 meet in the middle of the passage ; fusion of the two takes 

 place, and the new body, called as before the zygospore, 

 clothes itself with a cell- wall. It is liberated after a while 

 by the breaking down of the wall of the structure which 

 encloses it, and can then give rise to a new individual. 

 A similar process is characteristic of certain Fungi. 



In all these cases, though the cells are sexual cells, 

 the differentiation of sex is so slight that it is difficult to 

 speak of male and female gametes. In the Zygnemece, in 

 which the formation of the zygospore takes place in the 

 cell of the filament, the gamete which passes through the 

 passage may perhaps be regarded as male and the more 

 passive one as female. This differentiation cannot be dis- 

 tinguished in the Mesocarpece, where both gametes meet 

 in the connecting passage. 



In Ulothrix the differentiation of sex is even more 

 rudimentary, as it is not always necessary for the fusion 

 to take place. If any cell escapes fusion it may develop 

 into a new filament independently of this process. This 



