smith: application oi the theory of 



fche growing organ is thus a much more complex factor than 

 the temperature. 



Light. 



The other factor which must always be kept in mind is the 

 lighl intensity. There is a very general belief that light has 

 a distinct retarding effect on the growth of stems and roots. 

 This conclusion is based mainly upon the work of Sachs. 

 Baranetzky, Godlewski. and Wiesner. Sachs (15). upon 

 whose work, more than any other perhaps, the general conclu- 

 sion rests, certainly, as we have seen, did not distinguish 

 I iet ween the effects of darkness and increased humidity. The 

 difficulty of distinguishing between the effects of changes of 

 li^ht and changes of humidity is very great in the open air 

 and even in the laboratory experiments must be contrived 

 with this object fully in view before reliable results can be 

 obtained. Ii is therefore somewhat remarkable to find work- 

 ers ascribing changes of growth to alterations in light inten- 

 sity alone, when no records of changes of humidity were 

 la ken. 



Tims Baranetzky ( I) ascribes all his daily changes of growth 

 to alterations of lighl and darkness, although he makes no 

 records of alterations of atmospheric humidity. It is quite 

 true that in the laboratory changes of humidity are not 

 Likely to be so greal as outside, but in the absence of records 

 we cannot tell how greal or how small they may have been. 



Godlewski (6) using the epicotyl of Phaseolus multiflorus 



compares his results with those of Sachs (15) mi the daily 

 periodicity of stem growth. Sachs finds, as we have seen in 



discussing the case of Dahlia variabilis, a maximum growth 

 in the early morning and a minimum growth in Late afternoon 



in be U8Ual lor a certain nuniher of plants, and on this fact 



- his explanation with regard to the stored up retarding 

 effect of liyht. Godlewski, however, finds no regularity in 



the daily periodicity. He has sonic cases in which the maxi- 

 mum is in the afternoon and the minimum after midnight and 

 others which Bhovi 2 maxima and 2 minima per day. If the 



