9 2 



THE SCIENCE OF LIFE. 



generally found in still water. In an early stage of 

 growth the endochrome is diffused through their cavi- 

 ties, but after a time arranges itself in regular spirals. 

 Then adjacent cells put forth protuberances which coa- 

 lesce, and a passage is formed between the cells. The 

 endochrome of one cell passes over into the cavity of the 

 other, forming a sporangium, or spore-case. (Fig. 31.) 





FIG. 31. Various stages of the history of Zygnema quininum : A. Three cells, , , r, 

 of a young filament, of which b is undergoing subdivision : B. Two filaments in the first 

 stage of conjugation, showing the spiral disposition of their endochromes, and the protu- 

 berances from the conjugating cells. C. Completion of the act of conjugation, the endo- 

 chromes of the cells of the filament a having entirely passed over to those of filament , 

 in which the sporangia are formed. 



9. The most beautiful and interesting unicellular forms, 

 now generally conceded to be vegetable, are found among 

 the Diatomacece. Their motions caused many to regard 

 them as animals, but naturalists now agree in calling 

 them Protophytes. They are called Diatoms because 

 of their extreme brittleness and the ease with which 

 a chain of them may be broken into its component cells. 

 Like the Desmids, they are simple cells containing endo- 



