SUPPLEMENT 37 



Marine Algae must thus be complementary to those of their environment, 

 and hence this light, which is their optimal light, is absorbed by them. Con- 

 firmation of this view, which in our opinion, at least in relation to assimilation 

 itself, is by no means sufficiently well founded, is given by the interesting 

 results obtained by GAIDUKOW (1906). This investigator succeeded in inducing, 

 experimentally, in certain Algae an alteration in colour, and that, too, in the 

 sense of being a complementary coloration in relation to their surroundings 

 (Chromatic Adaptation, compare Lecture XXIV). STAHL also (1906) has 

 recently carried the matter a step further than ENGELMANN. He showed that 

 in nature the plant is not affected merely by direct sunlight, as is the case when 

 the heliostat is employed as a means of illumination, as is generally the case 

 in assimilation experiments ; for in addition the, quantitatively, by no means 

 inconsiderable amount of light arising from diffuse atmospheric reflection must 

 be taken into account. While, however, in the case of direct sunlight, the blue 

 and violet rays are absorbed in their passage through the atmosphere, it is 

 the red and yellow rays that are really lost in diffuse reflected light. According 

 to STAHL, the chlorophyll proper has for its function the absorption of the red 

 rays of direct light, while the accessory yellow pigment absorbs the blue rays 

 of diffuse reflected light. That the green rays are as little absorbed as the 

 ultra-red, STAHL explains in this way, viz. that these rays are present in so 

 small an amount in diffuse light that the plant is unable to make any use of 

 them, while they are so abundant in direct light as to injure the plant by 

 causing an excessive elevation of temperature. Thus the vigorous absorption 

 of light rays is prevented by the fact that the plant is not grey or black, but 

 green. We must await proof, however, of this view. It would seem to us 

 especially important to bring forward evidence of assimilation capacity in 

 blue light of chloroplasts of yellow colour only, not of green. We cannot 

 admit that KOHL (1906) has as yet proved this exactly. 



1. 44, for possesses read possess 



1. 45, for the end product, read an end product of combustion, 



1. 52, for 1847, read 1845, 



130, 11. 13-43, for It is possible . . . from that gas read It is possible to arrive 

 at an approximate estimate of these amounts in the first instance theoretically, 

 and for this purpose a knowledge of the following values is essential, viz. 

 (i) the amount of carbohydrate formed per unit of time per unit of leaf sur- 

 face ; (2) the heat of formation of these carbohydrates ; (3) the amount of 

 solar energy falling on a unit of leaf surface in a unit of time. The first value, 

 as we have seen, has often been estimated ; the second may be regarded as 

 equal to the heat of combustion ; the third has been calculated by physicists, 

 and more recently by BROWN and ESCOMBE (1905). These authors found that, 

 in the case of Helianthus, only 0-66-0-72 per cent, of the light falling on the leaf 

 was employed in photosynthesis. Similar results were obtained by PFEFFER 

 (1871) and AD. MAYER (1897) tne Y are ^ ess trustworthy, however, because 

 these authors did not determine, as BROWN did, the value of the incident light 

 in relation to assimilation. 



Only a very limited portion of the solar energy at least is used up in 

 photosynthesis. In addition to their theoretical determination an attempt 

 has been made to estimate their value experimentally. DETLEFFSEN (1888) 

 observed the absorption of light in a leaf with the aid of a thermopile, and 

 placed the leaf alternately in air containing CO 2 (10 per cent.) and air with 

 no CO 2 . 



1. 54, after solution, read As a matter of fact, much light is absorbed by 

 the leaf that is put to other than photosynthetic uses. According to BROWN'S 

 experiments the leaf of a sunflower exposed to a high temperature (27 C.) 



