Delf \ — Transpiration in Succulent Plants. 419 
piration was to be detected by means of the cobalt paper test ; this may be 
fairly interpreted as due to closure of the stomata prior to wilting. Other 
plants were brought into the laboratory and there the test was repeated. 
No such slowing down of the transpiration was then observed immediately 
after detaching ; and microscopic examination showed that the stomata on 
the whole plant had closed. I have myself ascertained by direct experiment 
that the stomata of Mesembryanthemiim edule closed within fifteen minutes 
after detaching a healthy leaf on which the stomata were initially open ; 
and in the case of Aster Tripolium and Plantago maritima the stomata 
closed in five to seven minutes after detaching a leaf. In neither of these 
cases could any sign of withering be detected until some time after the 
stomata had been observed to close. Moreover, in the case of Mesembry - 
anthemum edule , the leaf is well supplied with centrally placed aqueous 
tissue. When such a leaf withers the cells of the aqueous tissue gradually 
collapse, beginning from the innermost and slowly extending to the more 
peripheral cells. All the cells of the aqueous tissue show signs of collapse 
before the palisade tissue appears to suffer at all from want of water ; and 
the epidermis of a detached leaf only shows wrinkling after many hours’ 
exposure to drought, because the actual volume of the leaf is slowly 
diminishing. It appears to me impossible, at least in this case, to regard 
the closure of the stomata within the first fifteen minutes after detaching 
as due to ‘ the loss of water in the leaf as a whole 5 ; on the other hand, it 
seems probable that the closing of the stomata so promptly is, in this and 
probably also in many other cases, an adaptation which reduces the trans- 
piration of the leaf to the least possible under the particular physical 
conditions experienced at any time. 
How far the stomata of succulent plants are able to regulate transpira- 
tion is, however, a very difficult question. I am inclined to think that the 
limiting power of the stomata may be much greater than has been supposed. 
Even in the apparently closed condition of the guard cells some regulation 
of the size of the outer respiratory cavity may be demonstrated, as the 
following experiment with Salicomia shows. 
Two healthy similar shoots of Salicomia annua were chosen, and the 
cut ends sealed with a wax mixture. Both shoots were placed under the 
same conditions, one being used to determine loss of water during transpira- 
tion, and the other to determine the condition of the stomata during the 
course of the experiment. It was observed that the stomata appeared 
closed even before the shoots were cut ; the guard cells, however, showed 
a wide rift above the closed pore. The average dimensions of the stomata 
and the loss due to transpiration are shown in Table IV. At the end of the 
first hour the shoots were removed to a more shaded place, and this prob- 
ably accounts mainly for the diminution in size of the rifts, since, owing to 
the abundant aqueous tissue, an exposure of three hours could hardly affect 
