136 



Papers from the Department of Marine Biology. 



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, 

 Ipomcea, 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 coUenchyma 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 cUmates 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- 

 moea come next, while Tournefortia with its dense silky tomentum and 

 Sccevola with thick fleshy leaves hea^dly cutinized show relatively little 



