458 Prof. C. Timiriazeff. [Apr. 30, 



same amount of energy, which in the chloroplast is concentrated in a 

 thin film, was distributed in a mass of liquid thousands of times that 

 thickness. I do not mean that an experiment with chlorophyll in a 

 state of high concentration ought to give a positive result but it 

 might ; the experiment is worth making, and at all events the reverse 

 is not proven. At least I do not see any reason why chlorophyll 

 should act as a sensitiser on AgCl and AgBr, but lose its sensi- 

 tising power over C0_>. The difference is only quantitative, not 

 qualitative. 



Before dismissing the study of this intensity curve, I should like to 

 mention some applications which it seems to me might be made of this 

 law to certain facts concerning the geographical distribution and some 

 biological peculiarities of plants. It is a well-known fact that in high 

 latitudes plants require less time to arrive at the same stage of develop- 

 ment than in lower latitudes. This is generally attributed to the 

 greater length of the summer days. But then the sun does not stand 

 so high in those latitudes, and so long as the chemical effect was 

 considered proportional to the intensity of light, the explanation 

 was evidently unsatisfactory. Now that we know that only about 

 one-half of direct sunlight at noon is utilised, the remaining half 

 being of no use, it is easier to understand that a longer period 

 with a low sun is of greater importance than a shorter period with 

 high sun. 



We may take a step further and arrive at the conclusion that the 

 great intensity of the sun's rays at noon may not only be of no use 

 to the plant from the point of view of assimilation, but may become 

 directly injurious, as being the cause of an excessive evaporation, in 

 periods of drought even menacing its existence. Botanists are familiar 

 with many curious peculiarities of structure or habits of leaves, begin- 

 ning with the pubescence of the upper surface and ending with the 

 vertical position, the rib turned towards the zenith, permanently as 

 in the case of the compass-plant, or periodically as in many cases of 

 diurnal sleep. All these peculiarities are justly considered adapta- 

 tions reducing to a minimum the amount of evaporation. But one 

 might well suppose that this economy of evaporation could not be 

 realised otherwise than at a proportional loss of assimilation. In 

 order to escape suffering from thirst the plant might be exposed to 

 hunger. Now we may be sure that this is not the case, since about 

 half the direct sunshine at noon is sufficient to maintain nutrition at 

 its maximum. I have plotted (in calories) the diurnal energy curve 

 of Crova, the curve representing the quantity of energy incident on a 

 leaf placed horizontally, and lastly the double curve representing the 

 quantity of energy accessible to a leaf of the compass-plant with its 

 leaves placed in the plane of the meridian, the rib looking upwards. 

 If we remember again that only about half the energy at noon is 



