CHAPTER XIX. 



PTERIDOPHYTA. 



467. — The plants of this DiTision constitute the so-called 

 Vascular Cryptogams. They present an alternation of sexual 

 and asexual generations, much as in the Byrophytes, but in 

 the higher orders it shows signs of disappearing. The first 

 generation proceeds directly from the germination of the' 

 spore ; it is made up of simple tissues, and is usually short- 

 lived ; it bears the sexual organs, and hence is called the 

 sexual generation. The second generation, which results 

 from the fertilization of a germ-cell developed upon the 

 preceding one, is long-lived, and made up in most cases 

 of tissues of a high order, and the plant-body is differen- 

 tiated into root, stem, and leaves ; uf)on this second genera- 

 tion spores arise asexually year after year, and from these 

 spores the sexual generation is again produced. 



468. — The sexual generation, called the Prothallium, is 

 generally a flattened thallus-like growth, somewhat resem- 

 bling the plant-body of the lower Bryophytes. It is always 

 small, and composed throughout of parenchyma disposed in 

 one, or at most a few layers ; on its under surface it generally 

 produces root-hairs (rhizoids), which serve to fix it to the 

 ground, and doubtless also serve as organs of nutrition. 

 The cells of the prothallium are in most cases richly sup- 

 plied with chlorophyll, by means of which they elaborate 

 material for its growth. 



469. — When the prothallia have become sufBciently large, 

 they develop the sexual organs, the antheridia and arche- 

 gonia. These are formed in essentially the same manner as 

 they are in the two lower orders of Hepaticae (Ricci'acece and 

 Anthocerntem). They are more or less imbedded in the sur- 



