*774 



Botany. — "The priimiry photo-groiothreaction and the cause of the 

 positive phototropism in Pliycomyces nitens." By Dr. A. H. 

 Blaauw. (Communicated by Prof. F. A. F. 0. Went.) 



(Communicated in the meeting of December 27, 1918). 



Tliere are niimerouü investigations of the curvature movements, 

 which plants execute, when energy is supplied unilaterally in the form 

 of light or warmth, or when the organs of plants are displaced from 

 the positiou, which they Jiaturally occupy with resj)H't to gravity. 



The investigation of these "tropistic" movements, has proceeded 

 especially in recent years with greater precision, after it was found, 

 in 1908, that for the understanding of these curvature-phenomena it 

 is necessary to observe the effect of definite quantities of energy. But 

 whilst in this way more and more data have been collected, concerning 

 the curving of organs as a result of a unilateral action of energy, there 

 lagged behind all the more the study of the effect of energy, when 

 applied to the oi'gans not asymmetrically but radially. The occurrence 

 of curvatures as a result of asymmetrical forces, is a phenomenon 

 so striking, that it is easy to understand that much more attention 

 has been paid to the study of curvatures than to phenomena, which 

 occui- when the quantities of energy are supplied radially symmetric- 

 ally to the organs of the plant. Yet it is as a matter of fact more 

 natural to investigate first the influence exercised on an organ by 

 energy, such as light or warmth, when its action is distributed uni- 

 formly on all sides of that organ, and only afterwards to consider 

 as a special case what happens, when the energy reaches the plant 

 not equilaterally, but from one definite side. 



Now since it is a very important and well-known phenomenon, 

 that the plant reacts to this asymmetric energy in a striking manner 

 controlled by fixed laws, it was hardly conceivable that the plant 

 would not also react distinctly in the more general case in wiiich 

 the energy cicts radially symmetrically. With this equilateral action a 

 marked curvature was no longer to be expected, but it was possible 

 that any reaction taking place might not be limited to certain che- 

 mical changes in the cells, difficult to demonstrate, but might express 

 itself more clearly in a change in the rate of growth, which change 

 might be susceptible of measurement. 



After earlier investigations with light applied unilaterally, it seemed 

 to me desirable in consequence of the above considerations to pass 

 no further judgment as to the value and essence of curvature 

 reactions, until a further inquiry had been made into the way in 

 which a growing organ reacts when light, warmth or centrifugal force 



