Dar bishire * — Observations on Mamillaria e long at a. 403 
air-spaces do not favour a very rapid movement of the air in the plant But 
the air immediately outside the fleshy part of the plant is kept more or less 
stagnant by the passive action of the spines. The whole set of marginal 
spines are spread out and the spines of one tubercle overlap those of the 
next (PL XXV, Figs. 2, 3). The spines in fact form a fairly complete 
screen, separating the air which immediately surrounds the plant from the 
air outside. The former is to a certain extent stagnating. 
I have been unable to make any experiments to show that this is the 
case with Mamillaria elongata , but I have done so with Echinocactus 
cylindraceus , Eng. Placing a plant in the sunlight the temperature inside 
the spines and outside was measured. The temperature inside at three 
different times was 23 0 C., 28° C. and 28-5° C. The temperature outside was 
17-75° C, 18-75° C. and 19-75° C. respectively. This proves to me that the 
air inside was stagnating. It was close, and hotter than outside. The 
air inside the spaces will therefore not be rapidly renewed. The spines con- 
sequently have a very important function to perform : they reduce the rate 
of transpiration. Further observations were made, at the risk of losing 
a valuable specimen, in order to determine the temperature right inside the 
plant body, Echinocactus cylindraceus was again used, a thermometer 
being forced into a hole bored into the body of the plant. When the 
sunlight was falling directly on to the plant, the temperature inside the 
plant body was 15° C., in the space between the plant body and the screen 
of spines 19° C., and that of the air outside 16 0 C. The three thermometers 
had been in position i\ hours, the temperature outside the greenhouse in 
which the particular specimen of Echinocactus was growing being very low 
at the time. 
The air in the air-spaces is therefore evidently lower than that imme- 
diately outside the plant body, and we will clearly not get a rapid current 
of air outwards. The low temperature is clearly of use in reducing the rate 
of transpiration. 
Peirce makes the statement that the rate and volume of transpiration 
is reduced, in plants like the Cactaceae, by the body temperature of the 
plant being lower than that of the air when the air could otherwise take up 
most moisture ( 22 , pp. 137, 138). He refers for support of this view to 
Goebel, Schimper and Volkens. In the books of the two first authors 
mentioned I find reference made to a paper by Askenasy (12, p. 34, and 24 , 
p. 49). Askenasy gives a higher temperature for the inside of the plant 
than for the outside. This author enumerates a large number of plants 
where this is the case (2, p. 441). Volkens records an observation on the 
body temperature of Mesembryanthemum Forskalii. During the hottest 
part of the day the temperature of the leaf is 5 to 8° C. above that of the air 
( 33 , p. 40). It is a question about which a large number of readings need 
to be taken — for the Cactaceae at least — in the desert. 
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