TYPES OF CRYPTOGAMS; PTEEIDOPHYTES 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 (quiU-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. 



