96 SEX IN MICROORGANISMS 



grevillii (Ag.) Heib., Nitzschia palea (Kiitz.) W. Sm. (?), Rhabdo- 

 nema adriaticinn (Lyngb.) Kiitz., Sjirirella gemma Ehr. 



(b) Purely vegetatively. Bacillaria paradoxa Gmelin (?), Melo- 

 sira and other Centrales. 



DISCUSSION AND CONCLUSIONS 



In diatoms the most common method of reproduction is vegeta- 

 tive division by mitosis. However, there do occur other types of re- 

 production which have been discussed in this paper. As in the proto- 

 zoa, fungi, and other algae, it is hard to determine whether there is 

 true sexual differentiation. It is reproduction which occurs as a result 

 of meiosis and fusion. This type of reproduction has only rarely been 

 found in the Centrales. In the Pennales it has been found in numerous 

 species, but in most of them only rarely observed. This is particularly 

 apparent when one considers that there are several thousand species 

 of diatoms and that thousands of collections of diatoms have been 

 made. 



It is interesting to consider how this reproduction pattern, of 

 commonly occurring asexual reproduction with infrequently occur- 

 ring sexual reproduction, affects the structure of diatom species, their 

 distribution, and the evolution of the group. 



The common concept of the species is based upon populations 

 in which sexual reproduction is obligate and each individual has a 

 different genotype. Thus in a large population there is a great deal of 

 intergrading variation. In diatoms there are species composed of many 

 clones differing by sometimes small but disjunct variations. Each 

 clone consists of many individuals with the same genotype. It is per- 

 haps for this reason that we have commonly in diatom species so 

 many named varieties and forms. This difference in species structure 

 requires that any study of population structure based on random sam- 

 pling must be carefully planned with these facts in mind. 



This type of reproduction may also contribute to the distribu- 

 tional pattern of this group. Most species of diatoms have wider dis- 

 tributional patterns than are commonly found in higher plants. When 

 colonization is attempted by an obligate sexually reproducing species, 

 it is necessary for at least two individuals of opposite sex to be living 

 successfully in a given environment. In an asexually reproducing form 

 one individual may establish a colony. Thus, as Stebbins (1950) has 

 pointed out, asexual reproduction may bring about more rapid coloni- 



