THE HIGHER CRYPTOGAM I A. 133 



the youngest (already multicellular) leaf, to the place 

 where the stem ceases to increase in thickness (PI. 

 XVII, fig. 1). This cell-multiplication does not, how- 

 ever, occur exclusively in a specific group of cells, such as 

 is found in many vascular plants somewhat in the form of 

 a cylindrical envelope. It is no doubt true that the cells in 

 which division especially occurs are those of a conical 

 envelope lying underneath the outermost cellular layer of 

 the conical mass of cellular tissue. But the cells of the 

 inner layers are by no means passive (PI. XVII, fig. 1). 

 During these processes the cells of the outer surface divide 

 by radial longitudinal septa, and the cells of more vigorous 

 shoots also by tangential longitudinal septa, so that the 

 above peripheral cellular layer becomes transformed into a 

 double, triple, or quadruple layer of cells (PI. XVII, fig. 1). 

 In more slender shoots the latter form of cell- division is 

 suppressed : the cells of the periphery of the stem certainly 

 increase in number, by the formation of radial longitudinal 

 septa, whilst they keep pace with the increase of the peri- 

 phery of the axile cellular string ; the bark, however, re- 

 mains, for a time at least, a simple cellular layer (PI. XVII, 

 figs. 7, 9). The basal cells of the leaves, which are buried 

 to a certain depth in the tissue of the stem, and which are 

 easily recognisable by their peculiar tabular, flattened shape, 

 present in their ends, which are turned inwards, certain 

 indications from which it can be determined whether a 

 multiplication of the peripheral cells of the bark of the 

 stem has taken place or not (PI. XVII, figs. 1, 7, 8, 9). 



Whilst the growth in thickness of the stem is thus in 

 course of completion, its longitudinal growth is at a 

 stand-still. It commences with increased activity at the 

 spot where the conical form of the end of the stem passes 

 into the cylindrical form of the older portion. All the 

 cells become extended to at least twelve times their former 

 length, and during this elongation one more final process 

 of cell-multiplication takes place in them. The cells of the 

 interior of the axile string are often (although not with any 

 regularity) divided by transverse septa (PI. XVIII, fig. 8). 

 The cells of the periphery of this string divide still oftener 

 by radial and tangential longitudinal septa. They become 



