8o 



Elementary Biology. 



amount, until it ultimately cuts the old cell into two (fig. 

 20, ix.). The nuclei then take up a central position, and 

 the two new cells gradually assume the characters of the 

 parent cell from which they arose. 



Spirogyra does not differ essentially in its physiology from 

 the general type (Chap. VIII. sect. v.). The phenomena of 

 constructive and destructive metabolism are the same as 

 those exhibited by all green plants, save that its gaseous 

 food is obtained from carbonic acid dissolved in water. 



FIG. 2i. CONJUGATION IN Spirogyra, longata. (Sachs.) 



A, a, passage of the male protoplasm into the female cell ; f>, B, c, embryos 

 in different stages of development within the mother cell-wall. 



A species of sexual union and reproduction prevails in 

 Spirogyra, whereby new individuals are formed. Conjuga- 

 tion, as it is sometimes called, takes place between two cells 

 of different filaments lying close to each other (fig. 19). It 

 is to be noted that the conjugating cells are, to the eye, 

 precisely similar, although doubtless there are molecular 

 differences of importance ; the basis for that belief will be 

 given presently. The first stage in the union consists in 

 either cell sending out late v al protrusions or buds which 

 grow towards each other (fig. 19), and increase in length and 



