430 VEGETABLE PHYSIOLOGY 



ordinary individual, but can be made to do so by cultivating 

 them under adverse conditions of moisture, food supply, &c. 



A somewhat similar structure to the zoogonidia described 

 is put out by the coenocytic Alga Vaucheria. It appears 

 as a mass of protoplasm, which becomes separated from 

 the contents of a filament, and is set free by an opening at 

 the apex of the latter. It is composed of several proto- 

 plasts which are arranged together as in the rest of the 

 .coenocyte, but their individual outlines cannot be seen. 

 The fact that it is coenocytic is shown by the presence of a 

 number of nuclei in the protoplasmic mass. A pair of 

 cilia are given off opposite to each nucleus, so that it swims 

 very readily in the water after its liberation. It is some- 

 times called a Zooccenocyte. After a period of motility it 

 comes to rest, the cilia are withdrawn, and it becomes 

 clothed by a cell- wall. The resting period lasts for a variable 

 time, after which it develops into a new Vaucheria filament. 



Besides these asexual reproductive bodies other cells 

 are produced by the great majority of plants, which are 

 incapable of giving rise to new individuals, unless two of 



them unite or fuse with one 

 ,V 5 another. On account of this 



7* sft^' ff (/ ]/ <f* peculiarity they are known 

 tJ KJ ,i\|// A /jf^ as sexual cells or gametes. 



\ I ill// ^ %$ W/ ? 



In the lowliest forms, 

 such as many filamentous 

 AlgaB, they are produced by 

 the same filament as the 



Fiu. 169. - PART or- A FILAMENT or asexual Cells Or gonidia. In 



Ultfhrix FROM WHICH THE GAMETES the g^CIl Alga UlotJinX W6 



TfrTZeV ** gametes find the first "^Cation of 



conjugating. these sexual cells. Besides 



the large zoogonidia with 



their four cilia, other smaller free-swimming bodies are 

 developed in certain cells of the filament. They are pro- 

 duced in larger numbers and have only two cilia each (fig. 

 169). After they are set free into the water they swim 



