432 Delf . — Transpiration in Succulent Plants . 
In a few plants no difference could be detected, as in Rochea falcata, 
where all the leaves are fleshy, and in Phaseolus , where the leaves are 
mesophytic, the youngest leaves had a lower osmotic equivalent than the 
older ones. In these exceptional cases, however, there appeared to be no 
water transport from the older leaves to the younger during withering. 
Wiesner suggested that the water transport was due to the correlation 
between the young growing parts and the older leaves, the need of the 
actively growing parts forming a kind of mechanical check on the water loss 
of the older leaves by evaporation. Pringsheim remarks that this theory 
would not account for the fact that adult attached leaves transpire from the 
same fresh weight more strongly than similar detached leaves under the 
same conditions, and suggests that there is an internal regulating mechanism 
which adjusts the osmotic condition of the cell-sap throughout the plant, 
according to circumstances. In support of this theory, Pringsheim describes 
a number of experiments on the osmotic conditions of withering plants. 
Not only is there found to be what may be termed an ‘osmotic gradient’ 
in the normal plant, but this gradient is, in many cases, more or less main- 
tained during withering ; a few of Pringsheim’s results are quoted in Table X. 
Table X. 
Osmotic Conditions in Withering Plants. 
Fresh. 
% 
After one week's 
withering. 
°/ 
Tradescantia jhiminensis 
Y oung parts 
22-24 
Vo 
30-32 
Adult ,, 
10-12 
20 
Sempervivum tectorum 
Young ,, 
28-30 
34 
Adult ,, 
20-22 
30 
However, the most striking example of turgor-regulation was seen in 
the case of storage organs for food reserves, such as tubers. In the potato 
the osmotic pressure of the growing shoot was always greater than that of 
the tuber itself, but fell off from day to day as the reserve was gradually 
depleted, and in this way only as much starch would be utilized as would 
be needed for growth. The mechanism of turgor-regulation as conceived 
by Pringsheim consists in the manufacture of osmotically active substances 
in the actively growing regions, thus setting up a diffusion stream upwards 
which, however, can last only as long as the turgidity of the cells concerned 
is not wholly lost. 
It is probable that there may be some such regulatory mechanism 
within the plant, but this difficult question can hardly be regarded as 
wholly solved. As regards the question of the rate of transpiration of 
attached and detached leaves, whilst Pringsheim’s figures seem indisputable 
for transpiration measurements reckoned on fresh weight, yet, in experiments 
