THE PTERIDOPHYTES 



287 



purposes, and various kinds of parenchymatous tissue set 

 apart for different offices in the stem and leaves, ferns 

 have a higher vegetative organization than the simplest 

 seed-plants. The leaves are well furnished with stomata 

 and the work of photosynthesis is amply provided for. 

 JPteridophytes constitute the lowest 

 group of plants which have true roots. 

 The reproductive processes of ferns 

 are of a higher grade than those of liv- 

 erworts and mosses, but not ^s highly 

 developed as those of horsetails, club- 

 mosses, and Selaginella (Sects. 371, 374). 



EQUISETUM, A HORSETAIL 



367. Occurrence. — The commonest 

 horsetail, EquisetuTn arvense, occurs 

 widely distributed throughout the 

 United States. It is frequently found Pie. 210. A Water Fern 

 growing on railroad embankments and (Marsiha). 



on sand-hills. The fruiting stems ap- «, spore fruits, 



pear very early in the spring and are short-lived. The 

 sterile vegetative stems follow and become well grown 

 during June. 



368. Gross Structure. — Make use of herbarium sheets of plants 

 with fertile and with sterile stems, or sheets of one and fresh material 

 of the other. 



I. Study the position, form, and size of the rootstocks and the 

 places at which roots and stems arise from these. Are there any 

 attached tubers? 



II. Study the fertile stems, noting : 



(a) The shape of the stem as a whole. Is it solid or hollow ? 



