CHAPTER XIX. 



PTERIDOPHYTA. 



467. The plants of this Division constitute the so-called 

 Vascular Cryptogams. They present an alternation of sexual 

 and asexual generations, much as in the Byrf>phytes, 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 ; upon 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 sufficiently 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 (Ricciacece and 

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



