THE HIGHER CRYPTOGAMIA. 271 



strong shoots of Equisetum Umosum this increase takes place 

 with great regularity according to the progression 1 . . 4 

 7 . . 8 . . 9 . . 10 . . 11 and so on to 20 (PL XXXV, fig. 7). 

 At about the fourth or fifth leaf (reckoning backwards from 

 the uppermost youngest leaf) there occurs a very remark- 

 able longitudinal elongation of the cells of the apices of the 

 tip of the leaf. Even after its commencement, the multi- 

 plication of the cells of the base of the leaf in a longitudinal 

 direction by means of septa at right angles to the axis of 

 the stem continues for some length of time. 



The relation of the outer cellular layers of the stem 

 (which are produced by the development of the outer side 

 of the base of the leaf-rudiment) to the central pith cylinder, 

 which corresponds with the cells of the free portion of 

 the terminal bud, is very different in the different forms of 

 shoots. In the few vigorous shoots which are usually 

 developed in autumn by the subterranean internodes of 

 Equisetum palustre and pratcnse, and in a still more marked 

 manner in Equisetum Umosum and hyemate, the distinction 

 between the pith and the outer layer of the stem is visible 

 at a very early period : the cells of the latter even in the 

 youngest joints of the stem often divide repeatedly in a 

 longitudinal direction, whilst the multiplication of the pith 

 cells is quite at a stand-still. On the other hand this dis- 

 tinction occurs at a comparatively later period, and is not 

 nearly so well defined, in the delicate shoots which break 

 forth from the bases of the leaves high up on the stem, 

 especially in the thin shoots of the second, third, or fourth 

 order of Equisetum pratense, arvense and Umosum, or even 

 in the shoots of the first order of Equisetum variegatum. 

 The following table shows the number, in a longitudinal 

 direction, of the cells of the pith (a) and those of the 

 outer lavers (6). 



