280 
W. O. Howarth. 
4. The “ Radical ” Leaf. 
The outer epidermis of both sheath and lamina is composed 
of elongated cells, from twelve to fifteen times as long as broad, 
alternating with short cells. The short cells of the outer epidermis 
of the sheath may grow out as hairs (Pig. 10), or may develop 
into stomata, which are found especially in the cell rows close to 
the vascular bundles. The short cell divides longitudinally but 
unequally, and the larger of the two again divides longitudinally. 
Thus a central cell is cut off from two lateral ones. The latter 
Fig. 10. Subvar. tenuifolia. Fig. 11. Subvar. tenui/olia. 
Surface of sheath, x 120. Trans, sect, of lamina, x 120. 
become the subsidiary cells, and the central cell now divides 
longitudinally into two cells, each of which becomes a guard cell. 
The structure and mechanism of the guard cells has been fully 
described by Haberlandt (6, p. 451). The main differences between 
the sheath of the two forms are that in grandiflora there are about 
nine vascular bundles and a loose mesophyll; in tenuifolia only five 
to seven bundles, a compact mesophyll, and more thickened epi- 
and hypo-dermis. The outer sheaths decay and split, the vascular 
bundles being preserved intact by the protective endodermis and 
thus the lamina is enabled to continue its function for some time, 
afterthe sheath has split. 
