THE 



269 



381. Conjugation. Another method of reproduction is 

 by the formation of spores. In spirogyra and many other 

 algae the spores are formed by the method known as con- 

 jugation, that is, joining together. The cells of two adja- 

 cent filaments send out lateral protuberances toward each 

 other (Fig. 509), and when the ends of these protuber- 

 ances meet, the protoplasm in each contracts, the contents 

 of one pass over into the other, the two coalesce and form 

 a new cell but little, if any, larger than the original con- 

 jugating bodies. This cell germinates under favorable 

 conditions and produces a new individual. 



382. Diatoms and Desmids. These two groups are alike 

 in their microscopic size, in their simple structure, and in 

 the interest that attaches to them on account of their 

 enormous numbers and their great beauty and variety of 

 form, but otherwise they are not nearly related orders. 

 The diatoms are so different from all other vegetable 

 structures that they are 



i ., 



510 511 s S'3 



510-513. Diatoms (highly magnified): 

 510, 511, Grammatophora serpentina ; 512, 

 513, Fragllaria vircscens. 



placed by some bota- 

 nists in a class to them- 

 selves ; others group 

 them among the algae. 

 They consist of simple 

 cells inclosed in a very 

 hard mineral covering 

 formed of two valves, one of which fits over the other like 

 the lid of a pasteboard box. They are of a brown color 

 and of almost every conceivable shape (Figs. 510-513). 

 Not less than ten thousand species have been described, 

 and immense deposits of rock in various parts of the 

 world are formed by the flinty coverings of millions of 

 these microscopic creatures that once floated in the seas 

 of past geologic ages. 



The desmids were for a long time classed with animals, 

 but have now been handed over definitively to the botanist. 

 They are of a bright green color, and are further distin- 

 guished from the diatoms by their perfect bilateral sym- 



