124 Gibson . — Contributions towards a Knowledge of 
divisions parallel with the surface, forming a central core, which 
afterwards becomes the vascular bundle. At the same time 
the apex of the young leaf becomes elongated, and the 
cell-walls in this region become thickened. Division and 
growth now progress rapidly at the leaf-base, the cells of 
the lower surface being more active in this respect than those 
of the upper side. Hofmeister then describes the further 
modifications in the course of transition to the adult form, 
attention being drawn to the formation of marginal papillae 
and hairs, and the development of rows of warts on the cells 
near the margin of the leaf. He further points out that the 
median cells of the leaf are arranged so as to form a network 
with conspicuous intercellular spaces, whilst at the margins 
the mesophyll is wanting, and the upper and under epidermal 
layers are in contact. 
Russow 1 , in describing the leaves in the genus, speaks of 
the leaf as traversed by a single vascular bundle, which 
terminates before the apex of the leaf is reached. The xylem 
consists of spiral and reticulate elements, and is surrounded 
by a scanty phloem. No endodermis is developed. The 
ground-tissue, he states, consists of spongy mesophyll, well 
developed in the more robust species, but only surrounding 
the vascular bundles in those with more delicate leaves. 
Russow draws special attention to the epidermis of the leaf, 
and shows that it varies considerably both in the form and in 
the nature of the cells of which it is composed. In some 
species (e. g. N. Kraussiana and N. Galeottei) the upper and 
under epidermal layers are alike, and he suggests that pro- 
bably the leaves of all species belonging to the section 
‘ Articulatae 5 have this structure. Of Spring’s section ‘ Arti- 
culatae’ (which is not synonymous with that of Baker) I have 
examined only one other species in addition to those men- 
tioned by Russow, viz. S. sulcata, and in that species the 
upper epidermis is quite similar to the lower. In other cases, 
Russow continues, the cells of the ligular face are shorter 
1 Vergleichende Untersuchungen fiber Leitbundel-Kryptogamen. Mem. Acad. 
Imp. St. Petersb., 1872. 
