54 SCIENCE PROGRESS 



side, while the lower part and the part at which the cut was 

 made were kept in the dark. After six to ten hours the basal 

 part had curved towards the source of light. This most 

 interesting result indicates that it is not necessary to have 

 continuity of intact cells for the conduction of the stimulus. 

 The conduction of the stimulus is regarded by the author as 

 brought about by the diffusion of dissolved substances. This 

 view is similar to that held by a number of physiologists to 

 account for the transmission of the stimulation in the case of 

 the sensitive plant. 



The response of motile organisms to light has been the 

 subject of investigation by a number of workers. Miss R. 

 Bracher (" Observations on Euglena deses,'' Ann. of Bot., 33, 

 93-108, 1919) found that a Euglena that inhabits the mud of 

 the estuary of the Bristol Avon is visible on the surface of the 

 mud during the day, but disappears under the surface at night, 

 and that this phenomenon can be reproduced at will by placing 

 the organisms in the dark. The action of light on the movement 

 of a number of motile organisms was examined by Elizabeth 

 Bolte (" Uber die Wirkung von Licht und Kohlensaure auf die 

 Beweglichkeit griiner und farbloser Schwarmzellen," Jahrb. f. 

 wiss. Bot., 59, 287-324, 1920). These organisms are described 

 as positively and negatively photokinetic according as light or 

 darkness induces movement. The following were found to be 

 positively photokinetic : Chlamydomonas variabilis, Carteria, 

 some species of Euglena, Trachelomonas and members of the 

 Volvocaceae ; while among negatively photokinetic forms were 

 other species of Chlamydomonas , Hcematococcus, and Phacus. 

 Some species were found to be indifferent, among these being 

 Euglena hyalina, E, proxima and Chilomonas. R. Harder 

 (" t)ber die Reaktionen freibeweglicher pflanzlicher Organismen 

 auf plotzliche Anderungen der Lichtintensitat," Zeitsch. /. Bot., 

 12, 353-462, 1920) has investigated in detail the effect of change 

 i light intensity on the motion of Nostoc hormogonia. When 

 the light intensity is decreased there results a reversal of the 

 motion. For this reaction there is required a definite presenta- 

 tion time of the original stronger illumination, and also a 

 definite presentation time of the weaker light. These two 

 presentation times bear to one another an approximately 

 inverse relation, but not exactly so, the product of the two times 

 decreasing with increase in the value of the presentation time of 

 the original light. The intensity of the original light also has 

 an influence on the response, for with more intense illumination 

 the sensitivity of the hormogonia is reduced, so that there may 

 result an increase in the presentation time of the weaker light. 

 Thus if the intensity of illumination was increased and the 

 presentation time reduced so that the total quantity of light 



