90 STRUCTURAL BOTANY 



ture, and this tissue-system is the most constant and 

 characteristic feature in the anatomy. 



b. The Leaves 



The leaves of Equisetum are of little importance as 

 organs for gaseous interchange, and probably serve 

 chiefly as a protection to the lateral buds which arise 

 beneath them. However, they no doubt take a certain 

 part in assimilation and transpiration, as is shown by 

 their structure. These functions are of course limited 

 to the leaves of aerial shoots, and in the case of species 

 like E. arvense, to those of the sterile stems. 



The vascular bundles of the leaf-sheaths are of simple 

 collateral structure, and do not have carinal canals. 

 Each bundle is surrounded by its own endodermis, 

 whether this is the case in the stem or not. As in the 

 stem, the bundles correspond in position to the ridges 

 of the sheath ; outside each bundle lies a strand of 

 sclerenchyma. A narrow band of chlorophyll -containing 

 tissue lies between the sclerenchyma and the vascular 

 bundle, and approaches the surface on either side of the 

 ridge. The stomata are placed where the assimilating 

 cells reach the epidermis, so that there are two longi- 

 tudinal series of stomata corresponding to each vascular 

 bundle. The rest of the leaf -sheath consists of ordinary 

 parenchyma, which thins out between the ridges. The 

 teeth, which alone represent the free part of the leaves, 

 are still further simplified ; a vascular bundle enters each 

 tooth, but gradually dies out. 



c. The Roots. 



The roots of Equisetum are always very slender, and 

 must not be confused with the underground parts of 



