376 HOPMEISTER, ON 



cells of its diameter and circumference is increased by re- 

 peated division of the two outermost cellular layers of its 

 lower part, by means of radial septa, and of septa parallel 

 to the longitudinal axis of the stem. Near the place of 

 origin of the youngest leaf the longitudinal growth of the end 

 of the stem of Selaginella denticulata experiences a remark- 

 able acceleration, by the division of the cells of its outer 

 surface by means of horizontal transverse septa (PI. LIV, 

 fig. 8). 



The formation of the two youngest leaves commences at 

 a distance of from eight to ten cells — reckoning down- 

 wards — from the apical cell of the terminal bud. Two 

 horizontal opposite rows of cells, each of which occupies a 

 fourth part of the circumference of the stem, become arched 

 outwards, and divide contemporaneously by septa inclined 

 downwards (PL LV, fig. 21). In the outer three-sided 

 prismatic ones of the newly-formed cells, division then 

 ensues by septa inclined in opposite directions (PL LV, figs. 

 22, 23). The young leaf, viewed from above, now appears 

 as a narrow seam surrounding a fourth part of the circum- 

 ference of the stem (PL LIV, fig. V). It grows rapidly in 

 length by continual division of the cells of its fore-edge by 

 means of septa inclined alternately towards the upper or 

 the under surface of the leaf (PL LV, fig. 23 ; PL LVI, fig. 

 12). This multiplication of the cells is far more active in 

 the middle of the fore-edge than at its sides. The form of 

 the leaf would sooner become pointed Avere it not for the 

 fact, that the two middle cells of the fore-edge frequently 

 divide by longitudinal septa perpendicular to the surfaces 

 of the leaf and slightly diverging from its longitudinal axis. 

 In the young state of the leaf one such division almost 

 always occurs after each two divisions by septa inclined to 

 the surfaces of the leaf. Similar divisions occur from time 

 to time in the outer of the cellular groups near the middle 

 of the fore-edge of the leaf (PL LV, figs. 24, 25 j PL LVI, 

 figs. 5, 6). By this means the originally parallel arrange- 

 ment of the cells of the leaf becomes radiating and fan- 

 shaped. By the repeated division of all the marginal cells 

 of the leaf its base also becomes considerably widened. 

 The newly-formed basal cells do not amalgamate with the 



