SEXUAL AND ASEXUAL REPRODUCTION 239 



thus divided and separated from each other, each of the indi- 

 viduals grows until it is ready to divide, and so the process 

 goes on repeating itself. In most unicellular plants, the method 

 of reproduction is essentially the same. Figure 30, for example, 

 shows the reproduction in Pleurococcus, and Figure 33 in ordi- 

 nary bacteria. These latter plants are so small that we cannot 

 determine the internal changes that are going on, but can only 

 see that the individuals elongate and then divide in the middle, 

 into two parts. Recent study, however, seems to suggest that 

 the changes are essentially like those occurring in the Amoeba, 

 and at all events the process of reproduction is nothing more 

 than the process of division. 



The reproduction of yeast by budding (gemmation) is only 

 a modification of division; Fig. 32. The internal changes are 

 essentially like those in the reproduction of the Amoeba or 

 Paramecium; the first step is the division of the nucleus into 

 two, one of which passes out of the original cell into the bud, 

 while the other remains in the original cell. Thus, when the 

 two cells separate, each has a nucleus that has come from 

 the original nucleus, and, while the details of the process are 

 somewhat different, it is as truly a cell division as in the other 

 examples. Nearly all of the unicellular animals and plants 

 show one of these two methods of reproduction; see Fig. 111. 



Reproduction by Spores. When the organism breaks up 

 into many parts, they are called spores. Examples of this 

 we have already noticed among the unicellular organisms. In 

 the yeast (Fig. 32 s), spores are formed within the yeast cells 

 under some conditions; and Figure 25, which shows the life 

 history of the malarial organism, indicates that one part of 

 its history, namely, the cycle in the human blood, is an illus- 

 tration of spore formation. In the malarial Plasmodium the 

 spore formation which occurs in the human blood alternates 

 with a second type of spore formation in the body of the mos- 

 quito. This last process is, however, associated with celi 

 union, as shown in Figure 25 j. Among unicellular animals 



