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 collenchyma, 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 
