TYPES OF CRYPTOGAMS ; PTERIDOPHYTES 297 
species of Selaginella the leaves are arranged flat-wise on 
the stem, so that considered physiologically the branch- 
ing stem and its leaves together serve as a foliage leaf. 
In one of the commonest American forms, however, the 
stem is more nearly erect, and the leaves are all alike and 
four-ranked. 
Isoetes (quill-wort) grows attached to the soil in shallow 
water at the bottoms of ponds. It has the aspect of short 
grass growing in bunches. The large sporangia are at the 
broad bases of the leaves. 
367. High Organization of Pteridophytes. — The student 
may have noticed that in the scouring-rush and the club- 
moss studied there 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 structures deserve to be ranked just 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. 
