402 Dar bishire. — Observations on Mamillaria elongata. 
shallow. I need only refer to the work of Brown and Escombe (5) as 
showing that the small diameter of the air-spaces is rather favourable to 
the rapid introduction of the carbon dioxide of the air into the air-spaces 
of the green tissues than otherwise. 
The shallow air-spaces, however, serve a double purpose. The water 
brought up from the soil is evaporating into them, and the narrow diameter of 
the air-spaces has the important effect of reducing the rate of transpiration. 
Plants in dry regions reduce the lateral diameter of their air-spaces, 
although they cannot reduce their length, without interfering with the 
photosynthetical functions of the green cells. 
This then is the first instance, in our plant, of an adaptation to the dry 
locality, where the two vital processes, photosynthesis and transpiration, 
come into conflict. To suit the former they must be retained and must 
extend to as many cells as possible, to enable the actual absorption of the 
carbon-dioxide gas, which is always slow, to be carried out. To suit the 
latter they are reduced as much as possible in depth. 
The hypoderma, with its very few chloroplastids, no doubt serves as an 
additional protection, together with the epidermis, for the chlorophyll of the 
palisade-cells (PL XXV, Fig. 15). 
There are not many stomata. These, furthermore, show none of the 
many well-known xerophil characters (PI. XXV, Fig. 9). The stomata are 
on a level with the epidermis. The latter has not developed a very thick 
cuticle, and the outer cell-wall of the guard-cells is only a little thicker than 
that of the ordinary epidermal cells. 
In what way then does the plant protect itself against the strong and 
clear light, which forms such a feature of the Mexican desert ? Its harmful 
effect lies in its destructive action on the chlorophyll, and in the fact that 
strong light increases very rapidly the rate of transpiration. 
The whole cylindrical form of the plant is beautifully adapted to 
exposure to strong light. The sides of the tubercle get very little of the 
strongest light during the day. Their sides very rarely get the light falling 
directly at right angles on their surface (PL XXV, Figs. 2, 3). This is 
owing to the surface of the plant being elevated into tubercles. Strong light 
falls on to the plant generally at some angle, which will probably correspond 
on the average to the direction in which the rows of palisade-cells run. The 
light which falls on to the plant and actually reaches the body is therefore 
undoubtedly partly reflected in such a way as to be fairly evenly distributed 
over the whole surface. The result is important both from the point of 
view of the transpiration-stream and from that of photosynthesis. 
I have not, however, referred to the most characteristic external feature 
of our plant, namely the spines, which crown every tubercle. 
The rate at which transpiration goes on depends to a great extent on 
the rate at which the air in the air-spaces is renewed. The flat and narrow 
