256 PHYLUM IX. PTERIDOPHYTA 



which some are small (microspores) and the others large 

 (megaspores). 



454. In looking over the whole structure of the Ferns 

 it will be seen that the sporophyte has become the 

 dominant generation. This is due to the fact that in 

 its development it has pushed roots of its own down into 

 the ground from its lower end, thus insuring a constant 

 supply of water, while at the same time it has pushed 

 out some of the green tissue from its upper part into flat 

 expansions (leaves), thus insuring the supply of car- 

 bohydrates. The sporophyte has thus become in- 

 dependent of the gametophyte, and the latter, being now 

 useless after the maturity and disappearance of the sexual 

 organs, has become very short-lived, while the rooted 

 and leafy sporophyte has developed into a long-lived 

 plant, which may continue its growth for many years. 



455. With this longer life and larger size the fern 

 sporophytes have developed many kinds of tissues in 

 addition to parenchyma, including coUenchyma, scler- 

 enchyma, fibrous tissue, tracheary tissue, and sieve 

 tissue, some of which appear to be as highly specialized 

 as in the flowering plants. Furthermore, true vascular 

 bundles as well as bundles of fibrous tissue are developed, 

 the roots containing bundles of the radial type, and the 

 solid stems and leaves, of the concentric type. The 

 epidermis and stomata are scarcely to be distinguished 

 from those of the highest plants. 



456. The typically large leaves are sometimes simple, 

 flat blades, but more commonly they have branched into 

 "compound" blades of extraordinary complexity and 

 beauty of outline. The young leaves before expanding 

 are generally coiled or rolled, so that as they grow up 

 and open they unroll from below upward (i.e. cir- 

 cinately). Their vascular bundles (here usually called 



