412 SUMMARY OF CURRENT RESEARCHES RELATING TO 



a white image of the sun is obtained in the focus ; when the refran- 

 gible portion as far as the green is cut off, a red image ; and in the 

 same way a green or blue image can be obtained. In order to bring 

 exactly equal areas of the spectrum under observation, a scale is 

 placed exactly before the screen adapted to the dispersion of the 

 prism, and prepared therefore to suit each particular prism ; and 

 the screen is then put in position. The prism was made some- 

 times of flint-glass, sometimes of bisulphide of carbon; and the 

 action of the various rays of light was determined by measuring 

 the number of bubbles of gas given off in a unit of time from a shoot 

 of Elodea growing in water containing carbon dioxide. 



The result of the experiments is depicted by Eeinke in curves ; 

 the absolute maximum of evolution of gas was found to be un- 

 questionably between Fraunhofer's lines B and C, and nearer to 

 the former, corresponding to a wave-length of about A 690-680. 

 From this maximum the curve falls sharply towards the line A, 

 somewhat less sharply towards E, and from these more gently 

 towards H. If the absorption-spectrum of living leaves is compared 

 with this curve, it is seen that the maximum of evolution of gas coin- 

 cides with the absorption-maximum in the red, or with the absorption- 

 band I, while no secondary maxima of evolution correspond to the 

 secondary absorption-maxima II and III. The maximum of evolution 

 of oxygen, and probably also that of decomposition of carbon dioxide, 

 belongs therefore to those rays of the refrangible half of the spectrum 

 which are the most strongly absorbed by chlorophyll. 



From these facts Eeinke draws the conclusion that the action of 

 chloroj)hyll on the elimination of oxygen by plants is a chemical one ; 

 although the physical action of chlorophyll on which Pringsheim 

 insists is not altogether excluded, since the strong absorption of the 

 refrangible portion of the spectrum may be connected with this 

 physical function. 



Movements caused by Chemical Agents.* — W. Pfeffer has ob- 

 served that the motions of motile organisms and parts of plants are 

 to a large extent brought about by the exciting action of special 

 chemical substances, which, in very small quantities, exercise an 

 attracting influence. The substance which exercises the most powerful 

 influence in this way is malic acid, an acid very widely distri- 

 buted through the vegetable kingdom. It is the presence of malic 

 acid in the archegonium of ferns and Selaginellaceee which attracts 

 the antherozoids into the open channel ; while the specific attracting 

 substance for the antherozoids of mosses is cane sugar. The author 

 was unable to detect the attracting substance in the cases of Marsilia, 

 the Hepaticse, and Chara. In the Schizomycetes there is no one 

 specific attracting substance ; but any good nutrient fluid has this 

 power ; they will move towards the substance which supplies them 

 with most nutriment. 



The proportion of malic acid in the fluid in which the antherozoids 

 of ferns are swarming required to influence the direction of their 



* Ber. Deutsch. Bot. Gesell., i. (1883) pp. 524-33 ; also, Unters. aus d. Bot. 

 Inst. Tubingen, i. (1884); 120 pp. 



