TYPES OF CRYPTOGAMS; PTERIDOPHYTES 258 
are groups of leaves greatly modified for the purpose of 
bearing the sporangia. These groups are more nearly 
equivalent to flowers than anything found in the lower 
spore-plants, and the fern-plants which show such struc- 
tures deserve to be ranked not far below seed-plants in any 
natural system of classification. 
The variety of tissues which occur in pteridophytes is 
frequently nearly as great as is found in ordinary seed- 
plants, and the fibro-vascular system is even better devel- 
oped in many ferns than in some seed-plants. 
Starch-making is carried on by aid of abundant chloro- 
phyll bodies contained in parenchyma-cells to which car- 
bonic acid gas is admitted by stomata. In many cases 
large amounts of reserve food are stored in extensive root- 
stocks, so that the spring growth of leaves and stems is 
extremely rapid. 
