THE HORSETAILS 325 



saprophy tic in their life habits, being associated with certain fungi 

 which form a mycorrhizal partnership (Sec. 278) with them. 



CLASS II. THE HOKSETAILS, OR EQUISETINEJS 



316. The horsetails. The horsetails, or scouring rushes, are 

 all comprised in the genus Equisetum, which contains about 40 

 living species, the sole modern representatives of the order 

 Equisctalcs and the class Equisetineoe. These plants are the 

 remnants of what was a very extensive flora in an early geo- 

 logical period, called the Carboniferous Age, when the largest 

 deposits of coal were formed. The ancient relatives of Equi- 

 setum (Plate VIII, 2), together with the club mosses, were then 

 trees and formed the forests in those times. The horsetails live 

 now under what seem to be rather severe conditions in bare or 

 sandy soils that are unfavorable for the growth of trees, herbs, 

 and grasses. They illustrate very w r ell the way in which an 

 ancient group is sometimes able to avoid total extinction by 

 withdrawing as far as possible from competition with the 

 recent floras, and thus hold its own by means of peculiar life 

 habits and adjustments to special conditions. 



The most striking feature of the morphology of Equisetum is 

 the total absence of foliage suitable for vegetative activities 

 (photosynthesis). The foliage is represented by sheaths (Fig. 284, 

 A, B), which are found at the joints of the hollow stem. 

 The points on these sheaths are the tips of small leaves 

 more or less united below. The vegetative functions are per- 

 formed by the green stems. These are fluted, that is, they have 

 numerous ridges which run lengthwise, and the depressions 

 between the ridges contain stomata, which lie above the chloro- 

 phyll-bearing tissues (Fig. 284, F, c). The epidermis contains 

 deposits of silica, which is the chief substance in glass, and con- 

 sequently the stems feel rough. They are sometimes used for 

 scouring or polishing metal ; hence the origin of one of the 

 common names, " scouring rush." 



