THALLOPHYTES 23 



16. Sex-reproduction. A third method of reproduction 

 appears among the Algae, and it represents the final stage in 

 the progress of reproduction. This method was derived from 

 spore-reproduction, and some of the Algae illustrate this fact 

 completely. In Ulothrix (Fig. 7, B and C), for example, 

 a number of spores are produced by a single protoplast, 

 the number of spores depending on the number of successive 

 divisions. Naturally, the more numerous the divisions are, 

 the smaller are the spores, so that in Ulothrix the number 

 and size of the spores vary with the number of divisions. It 

 is found that the smaller spores produce feebler plants, and 

 that the divisions may become numerous enough to result 

 in spores too small to produce plants at all. Under these 

 circumstances it is observed that these small and incapable 

 spores may pair with one another and fuse to form a single 

 cell (Fig. 7, C, d and e), and that this cell can produce a new 

 plant. 



This act of fusing, by which a reproductive cell is formed, 

 is the sexual act, often called fertilization; the two fusing 

 cells, which are no longer spores because they cannot pro- 

 duce new plants alone, are sexual cells, usually called gametes; 

 and the resulting cell with reproductive powers is an oospore, 

 sometimes called the fertilized egg. It is evident that gametes, 

 among Algae, are derived from swimming spores, and that the 

 changes by which a swimming spore becomes a gamete are 

 the changes that explain the origin of sex. It is also evident 

 that the oospore is a spore, because it produces a new plant, 

 but it differs from the ordinary spore in the method of its 

 origin. It is for this reason that it is distinguished by a pre- 

 fix that means " egg," implying that it has been produced 

 by the sexual act. When the word "spore" is used, the 

 ordinary reproductive cell, not produced by the fusion of 

 two cells, is meant. Very often the phrases " asexual 

 spores " and " sexual spores " are used to distinguish these 

 two kinds of spores, but the latter phrase is misleading, for 



