EEPEODUCTION 



421 



duce gonidia when conditions are suitable for the life of 

 the 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 de- 

 scribed is put out by the coenocytic Alga Vaucheria. It 

 appears as a mass of protoplasm, which is 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 

 cosnocyte, but their individual outlines cannot be seen. 

 The fact that it is cosnocytic 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 sometimes 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 motility it comes to rest, the cilia are withdrawn, and it 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 another. On account of this 

 peculiarity they are known 

 as sexual cells or gametes. 

 In the lowliest forms, 

 such as many filamentous 

 Algae, they are produced by 

 the same filament as the 

 asexual cells or gonidia. In 

 the case of Ulothrix we 

 find the first indication of 

 these sexual cells. Besides 

 the large zoogonidia with 

 their four cilia, other smaller 

 developed in certain cells of 



FIG. 171. PART OF A FILAMENT OF 

 Ulothrix, FROM WHICH THE GAMETES 



g ARE ESCAPING. 



free gamete ; <7 3 , g 5 , gametes 

 conjugating. 



free-swimming bodies are 

 the filament. They are 



