240 Science of Plant Life 



evaporate, that the vegetative plants are killed by drying. 

 How, then, do they withstand the dry period ? 



It is resting spores that enable the plant to be carried over 

 the dry season. These spores are produced in large numbers 

 and, being heavier, than water, sink and become a part of the 

 mud bottom. When the pond dries up, the spores are in- 

 cased in the dried mud. At intervals they are watered by 

 rains, but they do not germinate for weeks or months after 

 their formation. No doubt a large part of the spores that were 

 formed perish during this period, but enough survive a drought 

 to start the plant the following season. 



Ulothrix. Another green alga occurring on the margins of 

 lakes, in running streams, and in clear springs is Ulothrix. 

 It has a filamentous body similar in many respects to Micro- 

 spora, and like that form it is attached to rocks and other 

 objects. Its methods of reproduction, however, are more 

 numerous and more complex than those of Microspora, and 

 they will serve to exemplify the reproductive processes of 

 many other forms of algae. When the filaments are mature, 

 the protoplasm within some of the cells divides into two, 

 four, or eight parts, each of which contains nucleus, cytoplasm, 

 chloroplast, and vacuole. Each of these parts becomes oval 

 in shape and develops into a swimming spore with four cilia. 

 An opening appears at one side of the original cell wall, and 

 a few minutes later the swimming spores pass out from the 

 cell cavity and swim away. Sometimes all the cells in a 

 filament produce swimming spores at about the same time, 

 and hundreds of these small green bodies may be found 

 moving about in the water. At the end of from 15 to 30 

 minutes the swimming spores settle down on some object 

 and become attached. By the end of a day the cell formed 



