448 
WRIGHT : FOLIAR PERIODICITY 
others. This plant possesses thick stiff leaves (which are pro¬ 
tected by a strong thick cuticle and a layer of cells running 
under the epidermis on the upper surface of the leaf) and 
narrow stomata. He proved that the transpiration from 
one plant of this species was, from the 23rd to the 27th 
January, both days inclusive, only 0*457 gram, or at the rate 
of about one gram in ten days. This means approximately 
one pound of water in 13 years, a very small quantity indeed. 
Alstonia scholaris was also experimented with. The 
leaves of this commonly occurring tree are not xerophytic ; 
they possess normal stomata andinternal tissues, anda cuticle 
of average thickness. One shrub of this plant possessing 100 
leaves lost during the 13th and 14th February, when the 
sky was cloudless and the temperature high, T24 gram ; 
but during the 15th and 16th when rain was falling only 
0*06 gram of moisture was lost by transpiration. Discus¬ 
sing the value of these and other observations Holtermann 
proceeds to state that there is between 12 and 2 o’clock on 
clear days in the tropics a much greater loss of water by 
transpiration than in Europe, though the total transpiration 
of a tropical plant in 24 hours may be less than in Europe. 
There are in the tropics a large number of plants which 
though exposed to the full rays of the sun only lose a small 
quantity of moisture by transpiration ; e.g., Euphorbia 
antiquorum, Cereus triquetra, and others. The leaves of 
these plants are small, and the green stems often carry on the 
greater part of the work of transpiration and assimilation. 
The stems of such plants are provided with a thick epidermis 
and deeply sunk stomata. On a very hot day in a very ex¬ 
posed place Cereus triquetra lost, according to Holtermann, 
from 8 A.M. to 6 P.M., only 0*01 gram per square decimeter. 
Holtermann also carried out experiments which proved 
that the tobacco plant lost twice as much moisture at Jaffna 
as at Peradeniya, and that the transpiration in the drier 
districts of north Ceylon is considerably greater than in 
the moist climate in the south-west of the island. He also 
states that though a change of external conditions did not 
