io MICROSCOPIC PLANTS. [CHAP. 



animal kingdom, but their title to be regarded as 

 plants has been long since thoroughly established. 



FIG. ii. 



FIG. 12. FIG. 13. 



They are, of course, exceedingly minute ; and it fre- 

 quently happens that in summer ponds are dried up, 

 and the Diatoms and other low plants are carried 

 long distances by the wind. We may then look in 

 vain among the dry mud of the late pond for active 

 Diatoms. We shall find what appear to be their 

 empty valves, the protoplasm having become dried 

 up by the sun. Yet if these are again placed in 

 water they will revive almost immediately, and as 

 soon as the rainy season commences we may search 

 in the refilled ponds and find Diatoms in their former 

 abundance. 



Long after the plant itself is dead the flint valves 

 retain their form, and consequently geologists find 

 vast beds and strata of rocks composed entirely of 

 them. The earth, called Tripoli, used in polishing, 

 consists almost entirely of their empty cases. Dr. 

 Hooker says : " The phonolite stones of the Rhine, 

 and the Tripoli stones, contain species identical with 

 what are now contributing to form a sedimentary 

 deposit, and, perhaps, at some future period a bed 

 of rock extending in one continuous stratum for 

 400 measured miles. I allude to the shores of the 

 Victoria Barrier, along whose coasts the soundings 



