HORSETAILS 159 



vegetatively developed. Two kinds of aerial shoots 

 rise from the rhizome, the one kind being fertile and the 

 other vegetative. The fertile shoots appear early in 

 spring; they are almost colourless, each one is sur- 

 mounted by a spore-bearing cone, and when the spores 

 are shed it dies down. It is a part of the economy of 

 the plant to waste no energy on shoots that have served 

 their purpose. The vegetative shoots appear after the 

 fertile ones have withered, and, unlike the latter, they 

 are branched, and the branches are of a deep green 

 colour. The fertile shoots reach a height of 6 or 8 inches, 

 whereas the sterile ones may attain a height of from 

 1 to 2 feet. It is the duty of these green shoots to 

 assimilate carbon, which is done principally by the 

 branches, all these having stomata and chlorophyll, and 

 to manufacture food material in excess of immediate 

 requirements, the excess being stored in the rhizome, 

 and forming a reserve enabling the fertile shoots to do 

 their special work in the following spring without 

 troubling about the commissariat. The fertile shoots 

 have no chlorophyll or stomata. The outer cell-walls 

 of the epidermis of the shoots contain silica, which 

 makes the surface hard. It does not happen in every 

 species that the fertile shoots are destitute of vegetative 

 capacity, as in E. arvense ; in some species, for example, 

 the Wood Horsetail, E. sylvaticum, spore-bearing cones 

 occur at the summits of ordinary vegetative stems. 



Still referring to Plate VII., we have in Fig. 1 a drawing 

 of fertile shoots of E. arvense, in which i is a leaf-sheath, 

 and a and b are cones composed of specialized leaves 

 called " sporophylls." These sporophylls bear sporangia 

 (e, f) on their under-surf ace. When ripe, the sporangia 



