HORSETAILS 205 



The structure of the small leaves borne on the 

 twigs has lately been worked out in great detail 

 by Mr. Thomas of Cambridge. They were per- 

 fectly organised little leaves, with a single vascular 

 strand, a starch-sheath, a tissue for assimilation, 

 a fibrous strengthening tissue, and an epidermis, 

 which bore stomata on the upper side of the leaf 

 alone. This unusual position of the stomata has 

 been explained by the probable hanging position 

 of the whole twig, which would turn the leaves 

 wrong way upwards. The cells of the stoma 

 show certain markings on their walls identical 

 with those observed in recent Equisetums. There 

 are few more perfect cases of the preservation of 

 structural detail than in these leaves. 



The roots sprang directly from the stem, not 

 from the bases of branches as in Equisetum. They 

 had in some respects the exact structure of Horse- 

 tail roots, but reached a much greater size, and, 

 like the stems, produced abundant secondary 

 tissue. An interesting feature is the presence of 

 large air-spaces in the cortex, a characteristic of 

 roots growing in water or very wet soil. 



We now come to the structure of the cones. As 

 already mentioned, they are, as a rule, more com- 

 plex than the cone of Equisetum. The latter con- 

 sists of an axis bearing shield-like fertile scales; 

 the Calamite cone, in all the best known cases, 

 consisted of an axis bearing shield-like fertile 



