218 BOOKS AND dUERENT LITERATURE 



they were water masses at a temperature about one degree higher than 

 that of the surrounding air. Since the experiments were carried out at 

 temperatures within about a degree of 15°C., it follows that these leaves 

 should have had a water vapor tension surpassing that of liquid water 

 similarly exposed, by somewhat less than 1 mm. of mercury (see tables of 

 vapor tension of water at different temperatures) . 



The relation which Darwin found may be generally expressed by the 

 equation x = y -\- 5 where x is relative humidity in percentage and y is 

 a number proportional to the transpiration rate. It seems to the re- 

 viewer that this mathematical statement is hardly complex enough to 

 necessitate the use of graphs at all in the discussion, though of course 

 the graphs give a ready means of picturing the general extent of the de- 

 parture of experimental from calculated rates of water loss. 



In the second paper, Darwin presents measurements which seem to 

 show that the lower ranges of light intensity (between darkness and 

 strong diffuse light) affect the transpiration rate in a marked way, aside 

 from any possible secondary effect brought about through stomatal 

 movement; greased and incised leaves give off appreciably more water 

 in weak light than in darkness. The acceleration due to light is shown 

 to amount, frequently, to more than 50 per cent of the rate in darkness. 

 No place is given to a discussion of the possible part here played b}^ varia- 

 tions in air currents about the leaves, it being tacitly assumed that these 

 are the same for both dark and light exposures. As to the causal condi- 

 tions controlling the generally increased transpiration in diffuse light, 

 Darwin writes, "we may either accept the view of Wiesner, viz., that in 

 light the chloroplasts are warmed by absorption of radiant energy, or we 

 may believe that light produces an increased permeability of the plasmic 

 membrane to water. " As has been pointed out above, the first alterna- 

 tive does not demand the supposition that the leaf is sensibly warmed, it 

 is only necessary that some molecules be given greater kinetic energy 

 (to resort to the kinetic picture) , and these may escape as vapor without 

 generally raising the temperature of the leaf. It is quite possible that 

 the second alternative may prove to be a consideration worthy of experi- 

 mentation; it has not been expressed before, so far as the reviewer is 

 aware. Of course an increased permeability of the plasmic membrane 

 to water must almost surely mean simply an increased swelling, with 

 higher water content; for such colloidal membranes transmit water 

 according to their content of this liquid and the more they resemble a 

 simple aqueous layer the more rapidly can water pass through them. 



These papers should stimulate further study of the physics of plant 

 transpiration. It should be strongly emphasized that further work 



