Sampson and Allen : physical factors on transpiration. 55 



less in these cultures than any in the series. Another fact that is 

 made clear ^is that for the types of soil investigated at least, the 

 amount of transpiration is correlated with the green weight of the 

 tops. 



Summary. 



A concise statement of all results obtained in this paper is given 

 as follows : 



1. Variation in transpiration per unit area for a given time is 

 foimd to be but slight, for plants of the same species, when about 

 the same age, grown and tested under similar conditions, whether 

 in their natural habitats or in the plant house. 



2. Of polydemic forms, when measured in their natural habi- 

 tats, those developed in the sun lose from two to four times as 

 much water as those developed in the shade. When the two forms 

 are placed in a sunny or shady habitat, the inequality of their tran- 

 spiration is about as manifest in the sun as in the shade. This in- 

 equality is largely explained from the fact that the sun form has 

 usually from twenty to sixty per cent more stomata per unit leaf 

 surface than the shade form. The greater activity which takes place 

 in the sun form also accelerates this function. 



3. Other things being equal, an increase in altitude stimulates 

 an increased transpiration. This acceleration is not due to an in- 

 creased light intensity and a lower air humidity as is generally con- 

 ceded, but is due to decreased pressure. This conclusion is further 

 substantiated by the fact that when plants were subjected to various 

 pressures under controlled conditions, the greatest transpiration took 

 place where there was least pressure. 



4. The transpiration of Scirpus laciistris, a ''bog xerophyte," is 

 almost twice as great as that of Helianthus anmius, a typical meso- 

 phyte with characteristic palisade and sponge tissue. This is read- 

 ily explained by the great amount of transpiring tissue to be found 

 in Scirpus. The external surface protected by an epidermis is fifteen 

 times less than the inner transpiring tissue. Of the total volume 

 of the stem, four-fifths is composed of air chambers. The number 

 of stomata is reduced, and the only xerophytic characteristic is its 

 leafless stem. 



5. Generally speaking, acid solutions accelerate transpiration and 

 alkaline solutions retard it, as compared with plants in distilled 

 water. In a very few cases extremely weak alkaline solutions were 

 found to increase transpiration. Transpiration often shows as 



