446 General Botany 



thought to have been derived. The product of photosynthesis 

 which accumulates most frequently in the ChlorophycecB is starch, 

 supplemented in many instances with oil. Both the starch and 

 the oil occur in greatest amounts in cells and parts of plants that 

 become reproductive structures. 



Multiplication occurs by cell division in the unicellular forms 

 and by the fragmentation of filaments in more complex forms. 

 Thick-walled cells are frequently formed from vegetative cells, 

 and these are highly resistant to drought and cold. Rounded 

 spores may also be formed by the contraction of the contents of 

 vegetative cells and the subsequent secretion of a new cell wall. 

 These spores usually remain dormant for weeks or months before 

 germinating and starting growth anew. In this way the plants 

 live through drought and winter conditions. 



Reproduction may also take place by swimming spores. These 

 are naked protoplasts, with two or more cilia, or flagella, that 

 vibrate and propel the cell through the water. A swimming 

 spore may consist of the entire content of a vegetative cell, or 

 several or many spores may be formed by several successive in- 

 ternal divisions of a vegetative cell. In any event they pass 

 out of the original cell wall and swim about for a few minutes or 

 a half hour, and then become attached to some object. They 

 then begin to divide and form new cells. 



If certain green algae are kept in the dark 12 to 24 hours, and 

 are then brought into the light, swimming spores will appear 

 in profusion in a half hour or an hour. Transferring algas from 

 cold water to warm water will frequently produce the same 

 effect. Swimming spores reproduce the plants rapidly and help 

 to spread them during favorable conditions. 



Sexual reproduction. The green algae present a remarkable 

 series of forms, with every gradation of sexuality. In sexual 

 reproduction the essential fact is the union of two gametes to 

 form a zygote. The gametes in the simplest forms are similar 

 free-swimming protoplasts, resembling swimming spores but 



