92 EVOLUTION OF LIFE, 
ance: though small in temperate regions, in the tropics it 
attains a size of fifteen or sixteen feet. The Horse-tail is 
composed of a series of hollow tubes joined end to end, 
the articulations being separable, and these tubes are 
marked externally by furrows running longitudinally. In 
place of leaves, the Equisetum exhibits green-colored 
branchlets; it has also rhizomes, or underground stems, 
sometimes extending to the depth of many feet. The 
spores are contained in a spike-shaped or conical cap, ter- 
minating the stem of the plant; the spores produce a 
ellular structure, the Prothallus, from which the new 
Equisetum will be developed: this kind of reproduction is- 
seen in the Ferns, of which we will presently speak. There 
is found in all parts of the Horse-tail such a large amount 
of silex that the plant becomes important in a commercial 
point of view, it being much used for polishing. 
FILICES 
Ferns are not only interesting to botanists on account 
of their structure and reproduction, but also equally attract- 
ive to the laity, their graceful stems and exquisite leaves 
furnishing specimens for the greenhouse and ornaments 
for the parlor. These beautiful plants are abundantly 
found in damp, shady places, though a damp soil and moist 
climate seem more necessary than shade for their luxuriant 
growth. If an oblique section (Fig. 123) of the stem of a 
Fern be magnified, the most important features observed are 
the vessels or ducts running down the middle of the stem, 
which have in them some woody tissue. So characteristic 
is the presence of vessels in the higher flowering plants, 
that Ferns, from having these organs, are often associated 
with them. At certain seasons there are seen, generally 
on the under surface of the leaf of a Fern, small bodies 
usually supported on stems, known as Sori. (Fig. 124, 2.) 
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