In this connection the actual transpiration records taken by the writer with a potometer in many of the representative plants of the Tortugas prove very illuminating. The records were taken in both sun and shade and are given in condensed form in the table following: It is seen that some plants have transpired the given quantity of 1 c.c. in a shorter time in shade than in sun. These are Suriana. Ipomoa, and Boerhaavia. This apparent contradiction of the law of transpiration, which expresses the fact that the higher the temperature the greater the water transpiration, is explained by the fact that in the intensely hot sunlight and the reflection and brilliant glare of the white sand those plants were heated beyond the point at which transpiration was at a maximum and the leaves lost so much turgescence that they wilted and became flaccid, with an almost complete inhibition of trans piration. This phenomenon is observed more particularly in young tissue, as tender shoots and branches which still have collenchyma in the stems and poorly developed epidermis on the young leaves. It was necessary to use such shoots of the three above-mentioned plants. since the older and more woody stems could not be fitted into the small Ganong potometer. However, this reaction was noted in plants grow ing in situ just as well as in the potometer subjects.
In several patches of Boerhaavia along a walk at the laboratory it was noted that during hot days, from before noon to mid-afternoon, the younger shoots would be hanging limp and flaccid, while about 4 p. m. they would assume their wonted erectness with turgescent leaves. Plants in more northern climates also frequently show this; notably Dahlias planted in open sunny places have often been observed by the writer to have all the upper leaves completely wilted in hot August weather during the middle portion of the day, while with the approach of evening they completely recovered. This temporary loss of turges cence may occur daily without apparent injury to the plants.
Of all the plants tested, Chamcesyce seems the best protected and transpires the given quantity in the longest time. Suriana and Ipo mcea come next, while Tournefortia with its dense silky tomentum and Smola with thick fleshy leaves heavily cutinized show relatively little difference in the rates in sun or shade conditions and actually give off a cubic centimeter of water in a very short period—i. e., in about 14 minutes. It is thus seen that this physiological action of loss of water varies considerably, even in these plants adapted to these xerophytic conditions and highly specialized morphologically.