194 PRACTICAL BOTANY 



from one cell to the other. Then the protoplasm from one 

 cell passes through the tube and unites with that from the 

 other cell. About this mass of protoplasm, which has now be- 

 come greatly condensed, there forms a very heavy cell wall 

 (Fig. 156). This heavy-walled body is a spore and may grow 

 into a new plant. It is formed by union of cells and not by 

 cell division, as was true in the case of the zoospores of plants 

 related to Pleurococcus. Reproduction by cell union is sexual? 

 and by cell division is asexual. The spore of Spirogyra is a 

 sexual spore. Because it is formed by the union of similar 

 cells, it is called a zygospore, which means a "yoked spore." 

 Cells that unite to form sex spores are called gametes ; hence 

 a zygospore is a sexual spore that is formed by the union or 

 conjugation of similar gametes. 



Dozens of Spirogyra cells in two adjacent plants may con- 

 jugate to produce zygospores. Usually the different pairs in 

 the united filaments will be found in about the same stage of 

 spore formation, though occasionally there will be some varia- 

 tion. Sometimes the cells from one plant will unite with cells 

 from more than one other plant. Also occasionally one cell 

 may unite with the adjoining one in the same plant. Some- 

 times the contents of cells that have not united with other 

 cells will assume the form and characteristic coverings of the 

 zygospore, and such spores may grow into a new plant. 



Zygospores are set free by the decay of the old cell walls. 

 After a period of rest, as during drought or winter, they ger- 

 minate and produce new Spirogyra plants. It is of obvious 

 advantage to the plant to have a heavy-walled resting spore 

 which will carry it through unfavorable periods. 



180. Cladophora: habitat and structure. Various species 

 of Cladophora are found attached to objects along shoals in 

 streams, over dams, about waterfalls, sometimes in heavy 

 floating mats along the margins of ponds, lakes, and even the 

 oceans. It is a most widely distributed genus, and one of the 

 few green algse ever found in salt waters. Cladophora usu- 

 ally has one end attached to some kind of support, and is 



