THE ALG^E 225 



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

 cell passes through this tube to the other cell, and the two 

 masses of protoplasm unite. The nuclei unite and one cell is 

 formed from what were two. About this new cell a heavy 

 wall is formed, and this specially formed structure is a spore ; 

 that is, a specially formed cell which can reproduce the kind 

 of plant that formed it. It must be noted also that this spore, 

 being formed by cell union and not by cell division, is sexual ; 

 also, sexual cells that are formed by union of similar cells are 

 called zyjiojywres, that is, yoked spores. It is customary to 

 speak of the cells that unite to form spores as gametes, and in 

 the case of the plant Spirogyra the gametes are similar, so 

 there need be no special name here for the two gametes, to dis- 

 tinguish them one from another. In asexual reproduction, as 

 will be seen in other plants, reproduction is carried on by means 

 of spores that are formed by cell division, not by cell union. 



In two Spirogyra plants there may be many cells uniting, 

 or conjugating, at the same time. These pairs are usually in 

 about the same stage of spore formation. Occasionally cells 

 from one plant will unite with those from more than one other 

 plant. Also, occasionally one cell may unite with the adjoin- 

 ing one in the same plant. When ripe, the zygospores are set 

 free by the decay of the old walls and may fall to the bottom 

 of the pond or stream. After a period of rest, sometimes after 

 a drought or in the spring, these spores germinate and pro- 

 duce new Spirogyra plants. It is obviously an advantage to 

 the plant to have a heavy-walled spore to carry it through 

 unfavorable periods. 



211. A branching alga: Cladophora. This is a green alga 

 of very wide distribution. It usually grows attached to objects 

 along shoals in streams, over dams, and about waterfalls. 

 Sometimes it appears in heavy floating mats along margins of 

 ponds, lakes, and even oceans. It is one of the few green 

 algae ever found in salt water. Cladophora is extensively 

 branched (fig. 176), and since its branched filaments are 

 abundantly supplied with chlorophyll, it is clear that this 



