98 
linear leaves later, I was surprised to find them tightly rolled into 
l he form of a straw, tapering to a point. Transverse sections of 
the leaves appeared as shown in Fig. A of Plate XVI. 
On the upper surface of the leaf are pronounced llattened 
ridges separated by furrows. A smaller more wedge-shaped ridge 
divides each main furrow into two secondary furrows. The section 
shows that the ridges are due to girders of selerenehyma, one being 
formed along each vascular bundle. The vascular bundle exactly 
resembles that of maize. The rnesophyll consists of a compact 
parenchyma, slightly differentiated into columnar cells on the 
lowe'r (outer) surface. Intercellular spaces are practically want- 
ing. A small air cavity exists behind each stoma, hut no other 
spaces are seen in a transverse section. There is therefore little 
cell surface exposed to an internal atmosphere and, consequently, 
evaporation is limited. On the other hand, no cell is more than 
three or at most four cells removed from the air cavity of a stoma 
and so can obtain by diffusion any carbon dioxide required for 
photosynthesis. The epidermis of the outer surface has a strongly 
developed cuticle and the lens-like enlargement of epidermal cells 
for light concentration and water storage is well shown. 
Chief interest) however, is centred about the abnormal epider- 
mis of the inner surface of the leaf. On the summits of the girder- 
like ridges the epidermal cells are small, frequently conical, or else 
produced into stout conical hairs. In the secondary furrows the 
epidermal cells are enlarged into thin-walled sacs, often conical, con- 
taining large water reserves and with bases in close contact with the 
compact rnesophyll. Cuticle is but slightly developed in the fur- 
rows. 
On excessive transpiration causing an undue loss of water, 
the enlarged epidermal cells (which we can call the “Curvature 
Tissue M ) in the furrows become less turgid, and shrinking, 
diminish the length of the epidermis stretching across the furrows. 
The width of the whole epidermis of the upper side of the leaf is 
diminished. At the same time the compact photosynthetic meso- 
pliyll contracts, water loss causing shrinkage of cells not separated 
by air spaces. Since the outer epidermis, protected and strength- 
ened by its tough cuticle does not similarly alter in dimension, a 
curving takes place, each furrow and the whole of the inner sur- 
face of the leaf becoming more concave. Should the water 
deficiency continue to increase the furrows are obliterated gradu- 
ally by the approach of the side walls and the whole leaf is tightly 
rolled. In plucked leaves this change takes place with great 
rapidity, curvature being marked in five minutes. This indicates 
that in the expanded position of the leaf transpiration is rapid, 
the plant by leaf movement being adapted to either dry or wet 
conditions. It is a plant well fitted for life in our climate of 
