THE VASCULAR CRYPTOGAMS 93 



them. The epidermis is also differentiated late, for 

 there is no distinct dermatogen near the apex. 



The whorls of leaves are at first crowded closely 

 together ; the internodes between them only begin to 

 lengthen some way down the stem. Each whorl arises 

 from the outgrowth of a ring of tissue which extends all 



FIG 42. Equisctum arvense longitudinal median section of the 

 apex of the stem, a, apical cell ; s, segment cut off from it ; 

 ^i> ^2> ^s> youngest leaves, in order of age. L, outline of 

 older leaves. Magnified 180 diameters. "(R. S.) 



round the stem. The circular ridge thus produced, which 

 is at first of equal height all the way round (see Fig. 

 42, ^ and 1 2 \ is the young sheath, and soon grows 

 out at certain places to form the leaf-teeth. We see 

 then that the sheath is formed first, and the free part of 

 the leaves later. 



The development of the branches in Equisetum is 



