THE DEVELOPMENT OF SEXUALITY IN PLANTS 173 



Another form of Oedogonium forms antheridia and oogonia on 

 separate filaments, the male filament being much smaller than the 

 female filament. Thus the filamentous algae illustrate three big 

 ideas, namely, division of labor, development of sex, and reduction of 

 chromosomes. 



In the simplest plants all cells tend to do the same work, but in the 

 more specialized algae there is a differentiation of work and an 

 accompanying differentiation of cells to accomplish it. In the 

 development of sex and of structures to take care of the sex cells, as 

 found in the forms described, the contribution of the sex cells seems 

 to be to provide a greater vigor to the offspring, especially when the sex 

 cells come from different individuals. Most important of all is the 

 fact that cells which fuse, as in the case of the sex cells, must have 

 some way of reducing the number of their chromosomes, else they 

 would be doubled each time two sex cells united. This is accom- 

 plished by the reduction division referred to above, by which process 

 the number of chromosomes, doubled at the time of fertilization, is 

 halved. This reduction process occurs in both plants and animals, 

 and although in plants it occupies a different place in the life cycle, 

 its ultimate effect is the same in both cases. 



A Representative Fungus 



Bread mold, Rhizopus nigricans, one of the most common of the 

 fungi, may easily be grown in the laboratory by exposing a moist piece 

 of bread to the air for a few moments. Mold spores are so numerous 

 everywhere that under ordinary conditions a growth of mold will be 

 evident within one or two days, first appearing as a white, fluffy 

 growth that rapidly covers the surface of the bread. This is the 

 mycelium, which consists of branching tubelike filaments, or hyphae, 

 containing many nuclei, but without cross walls. The absence of 

 chlorophyll shows the inability of the mold to make its own foods 

 and explains why the mycelium sends down into the bread, root- 

 like branches called rhizoids, that secrete enzymes, by means of 

 which the food substances in the bread are digested. Some of the 

 hyphae form long branches called stolons, which run along the sur- 

 face of the bread, forming new plants. At points where rhizoids 

 are developed, there arise later numbers of erect branches, or spo- 

 rangiophores, on the tips of which are developed sporangia, or spore- 

 bearing organs. 



