THE HIGHER CRYPTOGAMIA. 377 



neighbouring cells of the circumference of the stem, but 

 are developed independently, and form — when the leaf is 

 perfected — the basal appendages which are especially em- 

 ployed in the determination of the species. In the greater 

 part of its area the leaf of SelaginettaMartensi and 8. Galeottii 

 continues as a double layer of cells \ only the cells of a wide 

 longitudinal strip adjoining the median line on both sides 

 divide repeatedly by septa parallel to the surfaces of the 

 leaf, alternating from time to time with septa perpendicular 

 to the surface. Both divisions occur oftener in the inner 

 than in the outer cells of the longitudinal ribs which are 

 thus formed, and which project on the underside of the 

 leaf. The string of narrower cells which thus originates in 

 the longitudinal axis of the mid- rib is afterwards trans- 

 formed into a vascular bundle (PL LVI, fig. 11). 



Contemporaneously with the commencement of the thick- 

 ening of the mid-rib, a longitudinal expansion of the apex 

 of the leaf begins, an expansion, that is, of the middle cells 

 of the fore-edge, which by their more frequent divisions 

 have in the mean time shot ahead of the neighbouring cells. 

 The walls of these become thickened ; in the place of the 

 faintly-green mucilage which has hitherto filled the cells, 

 sharply defined chlorophyll granules make their appearance 

 in the fluid contents, which have become transparent. 

 After the commencement of these processes a multiplica- 

 tion of the superficial cells of the base of the leaf ensues, by 

 the division of such cells, once at least, by longitudinal 

 and transverse septa perpendicular to the surfaces of the 

 leaf. The cells of the under-side divide once oftener than 

 those of the upper-side, such division being produced by 

 septa at right angles to the longitudinal axis of the leaf. 

 In the perfect leaf the cells of the uncler-side are always about 

 one half shorter than those of the upper-side, which latter, 

 instead, divide once oftener by longitudinal septa, so that 

 they are only half as wide as those of the uncler-side (PI. 

 LVII, fig. 8). 



The double row of cells of the upper surface of the leaf, 

 which lies in the angle between the leaf and the stem, and 

 immediately adjoins the circumference of the stem takes 

 no part in these divisions. These cells remain consider- 



