LIMITING FACTORS TO GROWTH -MEASUREMENTS. ."{59 



daily periodicity therefore is not generally as Sachs described 

 it, his explanation of the facts as due to light effect, even if 

 true, must be of limited application. Godlewski himself 

 actually performed experiments in which the light intensity 

 changed, while he states distinctly the humidity was not 

 altered. In these the retarding effect of light was marked. 

 It is to be noticed however that he is himself unable to connect 

 the daily periodicity with the alterations of light intensity. 

 e.g.. he finds in June. 1888, a maximum growth in the after- 

 noon and a minimum after midnight a state of things which 

 is directly opposed to the supposition that light has a retarding 

 effect. He also finds that the daily periodicity is very irre- 

 gular at different times and in different individuals so that 

 not much value can be attached to his conclusion that the 

 daily periodicity depends on light conditions. 



Wiesner (22) conies to the general conclusion that light 

 retards stem growth and up to a certain limit favours leaf 

 growth. He remarks however that there are cases of stem 

 growth resembling leaf growth in this respect, and therefore 

 the retarding effect of light is by no means generally established 

 even granting the validity of those cases in which he supposes 

 the retarding effect of fight is shown. 



It seems to me quite probable therefore that the retarding 

 effect of light has been somewhat over-estimated. The most 

 satisfactory experiments on this question have been done on 

 seedlings, and it is doubtful whether a generalisation drawn 

 from these, which grow as a rule in more or less shady situa- 

 tions, should be applied to parts of plants normally growing 

 in full sunlight, such as the runner shoots of Capparis Rox- 

 burghii described in this paper. 



Whatever our judgment may be upon the experiments 

 which are supposed to have established the retarding effect 

 of light on growth as a generalisation applicable to all growing 

 organs, it is undoubtedly true that the light effect is com- 

 paratively small compared with those of temperature and 



