124 



THE THEORY OF EVOLUTION 



3. Pteridophytes or Ferns. Plants with stalk, 

 leaves, and true roots ; vascular bundles without ' true ' 

 vessels. 1 The alternation of generation is quite differ- 

 ent from that of the Mosses. The green plants (which 

 the layman only knows as the Fern) carry spore- 

 containing vessels (in which there is no ovum as 



A 



B 



FIG. 26. NEPHRODIUM filix mas. A Pro- 

 thallium from the under-side with archegonia ar ; 

 antheridia an; root-hairs rh. B Prothallus 

 with young fern produced from a fertilized ovum. 

 b, fern leaf ; w, root of same. Mag. about 8 diam. 

 (After Strasburger. ) 



with the Mosses). From the spores scattered on the 

 ground there are developed so-called prothalli, in 

 which true ova and seminal threads are formed. It is 

 only the fertilized ovum which reproduces the green 

 fern plant directly (Fig. 26). 



' True ' vessels (trachei) = conducting tubes in which the transverse 

 walls of the rows of cells which form such a tube become absorbed. By 

 this means the conduction of water is certainly facilitated and hastened. 

 In the ' Tracheids ' the transverse walls of the separate cells are retained. 

 That is the main difference. 



