100 



A TEXT-BOOK OF BOTANY. 



measures were chiefly laid down during this period. By some it is 

 thought that the deposits of coal of this age were probably princi- 

 pally formed from the remains of certain marsh plants including 

 two extinct groups of huge, tree-like club mosses (Lepidodendron 

 and Sigillaria) and the Calamites, representatives of the scouring 

 rushes. 



SPERMOPHYTES. 



The Spermophytes, or Seed Plants, constitute the third of the 

 great divisions into which plants are divided. The plants belong- 



FIG. 62. Successive stages in the germination of the microspores of a Selaginella: 

 p and w, cells of the prothallus; s, cells giving rise to sperms. A, B, D, views of spores from 

 the side; C, view from the back; in E, the cells surrounding the sperm mother cell are dis- 

 organized; P, two biciliate sperms. After Belajeff. 



ing to this division not only form the most conspicuous feature of 

 the flora because of their size and general distribution, but also 

 because of the fact that they produce flowers make a large num- 

 ber of them especially attractive. The plants of this group are also 

 of great importance from an economic point of view. They fur- 

 nish a large part of the food of man and other animals, as well as 

 materials for clothing, shelter, fuel, and divers other purposes. 

 In this group of plants there are the highest differentiation of tis- 

 sues and the most complicated structure. The one character which 

 especially distinguishes them from the lower groups of plants 

 is that of the production of seeds. 



