158 



FUNDAMENTALS OF CYTOLOGY 



discharging it between the protoplast and the cell wall. The nuclei 

 of the two gametes fuse, but their plastids do not. It appears that in 

 some species where only one of the gametes migrates the plastids of this 

 gamete degenerate in the fusion cell, leaving those of the other gamete 

 to continue into the next generation. This is more easily demonstrated 

 in the related genus Zygnema. As the zygote matures and becomes a 

 thick-walled resting zygospore, the fusion nucleus divides meiotically, 

 three of the four resulting nuclei then degenerating (Fig. 118). Upon 

 germination the zygospore develops into a new plant. Of frequent 

 occurrence in these genera is the formation of parthenospores, outwardly 

 resembling the zygospores, by single cells without fusion. 



Chlamydomonas, a unicellular green alga, is also a haplont. The 

 motile vegetative cells under certain conditions unite two by t^^•o as 



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Fig. 116. — Development of blepharoplast in zoospore of Ocdogonium. (After H. Krctschmer.) 



gametes. The resulting zygote is diploid, and when it germinates meiosis 

 occurs, each of the four new motile cells arising from it having the reduced 

 chromosome number. These organisms, which can be grown in large 

 numbers in a small space, have been the subject of a number of genetical 

 researches. The inheritance of characters exhibited by the vegetative 

 cells and zygotes are as should be expected in an organism with this type 

 of life cycle. 



A second general tj^pe of life cycle is represented in certain species of 

 Cladophora and other genera. In C. suhriana, for example, there are two 

 kinds of plants that look but do not behave alike. The plants of one 

 type arise from zoospores, have the monoploid chromosome number, 

 and produce biciliate gametes. This species is heterothallic. After 

 the union of two gametes, the resulting zygote grows into a diploid 

 plant which bears monoploid zoospores. Hence this cycle shows a well- 

 marked alternation of generations, and species having it are termed 

 diplohaplonts. 



A third general type of cycle is found in Cudium and certain other 

 genera of the coenocytic Siphonales. These plants show no alternation 

 of generations and are diplonls, the nuclei being diploid throughout the 



