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. 
Pteridophytes 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 as highly 
developed as those of horsetails, club- 
mosses, and Selaginella (Sects. 371, 374). 
EQUISETUM, A HORSETAIL 
367. Occurrence. — The commonest 
horsetail, Equisetum arvense, occurs 
widely distributed throughout the 
United States. It is frequently found Fis. 210. A Water Fern 
growing on railroad embankments and ee): 
on sand-hills. The fruiting stems ap- ee 
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 ? 
