84 SUMMARY OF CUREENT RESEARCHES RELATING TO 



insects, compared with those not so fed under similar circumstances, 

 even when an abundant supply of a nutrient fluid was furnished to 

 their roots. 



Mechanical Action of Light on Plants.* — F. Cohn has investi- 

 gated not so much the cause of the apparently spontaneous movements 

 of the lower plants and of animals, as the forces which induce those 

 movements to assume certain definite directions. 



Non-chlorophyllaceous organisms, such as monads and the zoo- 

 spores of fnngi, move freely in every direction indifferently in refer- 

 ence to the incidence of the rays of light.| Diatoms and Oscillarieje, 

 coloured respectively by phasophyll and phycochrome, always prefer 

 light to darkness, aud accumulate therefore on the surface of the 

 water. When the field is equally illuminated in all directions, diatoms 

 are distributed uniformly through the water, and Oscillarieas radiate 

 equally in all directions. Green microscopic organisms which contain 

 chlorophyll, such as Eugleneas, Yolvocinese, and the zoospores of most 

 algae, always display a certain polarity, one end being destitute of 

 chlorophyll and usually provided with cilia and a red " eye-spot," and 

 being also more pointed in comparison to the other end, which is 

 coloured a deep green. The pointed end is always the anterior end 

 in the " swarming " motion ; and this advancing motion is always 

 accompanied by a rotating movement round the longitudinal axis 

 which passes through the two ends ; the direction of this rotation 

 varies in different organisms. A number of experiments undertaken 

 by Cohn show that when the direction of the incidence of the light on 

 the field of view is made to vary, the direction of the motion of these 

 green organisms varies with it ; they always seek light and avoid dark- 

 ness. But it is a remarkable fact in connection with this, that it is the 

 direction rather than the intensity of the light that seems to influence 

 them ; as is seen when the light is reflected on to the field of view from 

 a mirror. Reflected light appears to have no more effect in influencing 

 the direction of their movements than absolute darkness. Experiments 

 with coloured glasses show that it is only the more refrangible 

 actinic rays which have this effect on the movements of minute 

 organisms ; the less refrangible, which have no chemical action, have 

 also no effect of this kind. A few exceptional organisms display a 

 power of motion in the opposite to the ordinary direction. 



A comparison of these movements with those of artificial euglenas 

 which are made to evolve carbon dioxide from one end, shows that the 

 direction of the movement is dependent on the decomposition of carbon 

 dioxide by the aid of the organism which contains chlorophyll, and 

 hence on its polarity. 



These movements of green swarm-spores and similar bodies are 

 compared by the author with the phototonic movements of the organs 

 of plants, on which many observations have recently been made, 

 especially by Stahl.| 



* JB. Schles. Gesell. Vaterl. Cult., 1883, pp. 179-86. 



t With the exception, however, of bacteria, as shown by Engelmann. See 

 this Journal, ii. (1882) pp. 380, 656 ; iii, (1883) p. 256. 

 X See this Journal, ii. (1882) p. 373. 



