THE VEGETABLE KINGDOM 333 



female gametophyte, which is fertilized and produces an 

 embryo of a new sporophyte all without ever being shed 

 from the megasporangium, which, when all these develop- 

 ments have taken place within it, is called the seed. The 

 microspores are called pollen, and germinate on the 

 megasporangium. Thus the dependence of the gameto- 

 phyte on an aquatic (or even a damp) environment is 

 entirely avoided, and adaptation to the driest conditions 

 becomes possible. 



Algae. Most of the Algae leave no trace in the 

 rocks except indefinite carbonaceous remains. There 

 are two groups which form exceptions to this rule. 



1. The Diatomacece (diatoms), microscopic forms with 

 a siliceous skeleton, with a very fine network-structure, 

 resembling that of Radiolaria but much finer. These 

 occur in immense numbers in fresh and salt waters, and 

 their remains accumulate to form masses of diatomaceous 

 ooze, in the cold Arctic and Antarctic seas especially. 

 Similar deposits occur in Tertiary and Cretaceous rocks, 

 and are worked for polishing powder and as the inactive 

 basis of dynamite. The deposits of Bilin (Bohemia) 

 and Richmond (Virginia) are examples. Sometimes the 

 deposit becomes a chert. Below the Cretaceous rocks 

 they are practically unknown. 



2. Among the red and green seaweeds there are a 

 number of families in which calcium carbonate is 

 deposited in the cell-walls. These forms are collectively 

 known as "calcareous algae," and some of them have 

 been shown to play a large part in the building of 

 modern coral-reefs, and they were no less important as 



