ALG^ 367 



arrangements of their cilia, while some spores of algae have 

 no cilia at all. Swimming spores are called zoospores 

 {animal-spores). 



The most interesting fact about Ulothrix is that in it we 

 find the simplest form of sex reproduction. The number 

 of spores produced by a single protoplast is not a fixed 

 number. Sometimes the protoplast goes on dividing until 

 it forms as many as thirty-two spores. The greater the num- 

 ber of spores, the smaller they are. As the spores become 

 smaller, their power to produce new plants appears to become 

 reduced. Though the larger spores germinate directly into 

 new plants, the smaller spores have been observed to unite 

 in pairs before germination occurs. It appears that these 

 spores, too feeble to produce new plants alone, unite their 

 forces, and the cell resulting from their union accomplishes 

 what the separate cells could not. United, such cells can re- 

 produce ; separate, they cannot reproduce. Such cells, hav- 

 ing lost the power to reproduce independently have thereby 

 lost. the right to be called spores. They are called gametes. 

 Cells which cannot reproduce independently, hut which can by 

 fusion produce a cell that reproduces, are gametes. Evidently, 

 wherever there is a sex process there are gametes. (Study 

 Figure 164.) Ulothrix produces both spores and gametes. 

 The eggs and sperms of which you have already heard are 

 evidently gametes. The fusion of the gametes of Ulothrix, 

 just as the fusion of sperm and egg, is fertilization. The 

 cell which results from the union of the gametes is called 

 the oospore {egg-spore) or fertilized egg. 



Thus we see that gametes are derived from swimming 

 spores, and that this evolution of a spore into a gamete 

 is the evolution of sex. Remember that cells which must 

 fuse before reproduction occurs are gametes, and cannot 



