233 
Leaf Water-content , and Transpiration Rate . 
and 49). Shreve’s suggestion — that incipient drying checked transpiration, 
and produced the early maximum, thus allowing the water-supply to catch 
up with the demand and causing an increase in the transpiration rate up to 
the second maximum— is clearly inadequate. 
There is considerable discrepancy between the results described above and 
those obtained by other workers. Livingston (17) and Shreve (21), and also 
Trelease and Livingston (22), found the maximum of relative transpiration 
to occur regtilarly before that of evaporation. Also Trelease and Livingston 
report this transpiration maximum as occurring before the maximum of sto- 
matal aperture, whereas from the present work there appears to be no constant 
relation between the time of occurrence of these three maxima. The explana- 
tion may be found in the difference between the prevailing climatic conditions, 
some of the present experiments having been carried out on very bright days 
and others on very dull days. Bright days, with a correspondingly high 
evaporating power of the air, will naturally favour high transpiration rates 
and incipient drying, whilst on dull days the transpiration rate will be lower 
and may not exceed the rate of absorption. Both Livingston and Shreve 
carried out their experiments at the Desert Laboratory at Tucson, where the 
relative humidity of the atmosphere was very low and the temperature 
fairly high — conditions favouring the early onset of incipient drying and so 
producing the early maximum of relative transpiration. 
Stomata and Leaf Water-content. 
Lloyd (20) found that frequently the stomata continued to open when 
the water-content of the plant was decreasing, and concluded from this that 
the stomata were * ineffectual in maintaining a constant supply of leaf- 
water \ The results of Trelease and Livingston mentioned above, showing 
that the stomata continue to open for some three hours after the transpira- 
tion maximum is reached, confirm Lloyd’s experiments, since the water- 
content of the leaf must decrease with the onset of incipient drying which 
occurs before the maximum of relative transpiration is reached. In other 
words, the stomata continue to open whilst the water-content of the leaf is 
decreasing. 
The same result has been found in the present experiments. When the 
water-content of the plant is decreasing owing to the excess of transpiration 
over absorption, the stomata often continue to open, thus tending to increase 
the rate of transpiration. 
Experiment 1 3 quoted above provides an example of this. Transpiration 
exceeded absorption (and therefore the water-content of the plant decreased) 
from the commencement of the experiment at 11.0 a.m. until 3.30 p.m., but 
the stomata continued to open until 13.15 p.m. A further example is given 
below. 
