351 HOtfMEISTER, ON 



eighth, exhibit the \ arrangement.* The commence- 

 ment of the formation of each new leaf takes place some 

 time before the cessation of the growth (i. e., the cell-mul- 

 tiplication of the base) of the next preceding one. The 

 lowest part of the back of the leaves enters into the forma- 

 tion of the bark of the stem, like the base of the underside 

 of the leaf of the Equiseta, the Lycopodiacese, and the phae- 

 nogams. The I arrangement of the leaves causes an 

 excessive increase in the mass of the cortical tissue at two 

 points of the circumference of the stem lying opposite to 

 one another, and corresponding with the dorsal surfaces of 

 the leaves. Here the bark is developed so as to form two 

 fleshy bodies widened at the base,f and spreading obliquely 

 downwards from the ligneous body of the stem. These 

 bodies are separated from one another by a flat indentation 

 which is at right angles to the large horizontal axis 

 of the stem, and is the first rudiment of the characteristic 

 furrow of the underside of the stem. 



The development of each young leaf, and the growth in 

 thickness of its base, proceed pari passu with the cell-multi- 

 plication in circumference and diameter, of the older 

 portion of the end of the stem, as well as of the base of the 

 preceding leaf by which the young leaf is sheathed. The 

 active increase in the number of the cells round the upper 

 end of the longitudinal axis, pushes the previously formed 

 portions of the circumference of the stem continually 

 further outwards. The latter are able to bear this pressure 

 for a long time by the expansion of their cells in a tangen- 

 tial direction. But in the plane which passes through the 

 small horizontal axis of the stem this expansion is almost 

 entirely suppressed. At this part the cortical parenchyma 

 splits from the outside (sideways and from below), at an 

 early period, by which means the furrow of the under side 

 of the stem is made much deeper. 



Shortly after the commencement of the formation of each 

 new leaf, a new root is produced laterally beneath it ; the 



* As Alex. Braun remarked in 1847 ('Flora,' p. 135), and which he pointed 

 out as the immediate cause of the bipartite arrangement of the stem of Isoetes. 

 f This widening is due to the greater vigour of each new leaf. 



