Delf . — Transpiration in Sticculent Plants. 439 
which enables the whole plant to achieve its period of growth in the 
shortest time, the rapid assimilation compensating to some extent for the 
short summer. A somewhat greater transpiration value was found for 
alpine than for corresponding lowland forms, both relative to surface and to 
fresh weight ; and this in spite of the fact that on the whole there was more 
cuticular and hairy protection afforded to the alpine types. The greater 
transpiration, however, is probably due to the relatively greater number of 
stomata which was found to be a characteristic feature of alpine plants, and 
to the direct effects of increased assimilation and respiration. 
Observations of Holtermann 1 on the vegetation of mountain peaks 
in Ceylon indicate that here also something more than mere altitude must 
be responsible for the characteristic modifications which were found. 
According to Holtermann the peculiarities associated with alpine plants 
only appear in Ceylon on mountain peaks over 5,000 ft. in height, but not 
at all in the vegetation of plateaus of an even greater altitude. The most 
obvious source of explanation was stated to be the absence of humus in the 
rocky districts of the mountain summits, although possibly other climatic 
factors might be concerned. If this explanation be true for the alpine 
vegetation of Ceylon, it is, however, scarcely applicable to that of 
Switzerland, and it is not borne out by the experience of horticulturalists 
who now succeed in growing even exclusively alpine plants in suitably 
prepared rockeries, the interstices of which contain usually a rich humic 
soil. 
Aubert 2 attempted to compare by direct measurement the rate of 
transpiration relative to both the transpiring area, and to the fresh weight in 
succulent and mesophytic plants. The method employed was, admittedly, 
rough, and is open to criticism, but the general conclusion is probably trust- 
worthy, that the Cactaceae transpire the least of any succulent plants as yet 
tested, and that next in order of magnitude come succulent plants of the 
type of the Crassulaceae and the Mesembryantheae, which have, themselves, 
a greater rate of transpiration than many sclerophytes, such as Picea and 
Nerium. In these latter types, however, there may be a very large collec- 
tive leaf surface, so that the total water loss may be very much greater than 
in more succulent plants. We have seen that in halophytic, epiphytic, and 
alpine plants the rate of transpiration may be distinctly greater than in the 
average mesophyte. Such observations as I have made indicate that in our 
British Crassulaceae, and in those Mesembryantheae which have become 
acclimatized, the transpiration is greater relative to the surface area than in 
ordinary mesophytes, although the latter usually possess more numerous 
stomata. The transpiration relative to the initial fresh weight is, naturally, 
less in succulent than in mesophytic plants. The explanation of the 
1 Holtermann, Dr. C. : Der Einfluss des Klimas auf den Bau der Pflanzengewebe, 1907. 
2 Aubert, E. : Turgescence et transpiration des plantes grasses. Ann. des Sci. Nat., Bot., 1892. 
G g % 
