76 



of cases, of whioh the Monrovia data are a good example, in 

 which the light appears to clay a definite, though secondary 

 part. That it is playing such a part is indicated by the fact 

 that the stem height graph, while corresponding to the tem- 

 perature graph in general form, shows minor variations in slope 

 noted in the course of the discussion which suggest an inverse 

 relation between stem height ind sunshine intensity. Another 

 objection to the interpretation lies in the fact th t at the 

 end of the season ..hen sunshine intensity is very low the 

 rate of stem elongation falls off very rapidly. Thip is not 

 what we would expect on he assumption that li ght and stem 

 elongation are inversely proportional. In ans^^/er to this, it 

 may be said that evidence can be secured from the data tending 

 to show that the low light values at the end of the season 

 depress the rate of photosynthesis in the plants. This evi- 

 dence will be giv n in a later paper. If the rate of photo- 

 synthesis is lowered it is reasonable to suppose that there 

 may be a defJ.ciency in the amount of nutrient available for 

 growing regions, resulting in a diminution of the rate at 

 which the various growth proce sses take place, T?e -^hus se- 

 cure a possible explanation for the relatively great falling 

 f in height, ( and in the other growth ra'^es, as well^ occurr- 

 ing at the end of the season. A third objection to the above 

 interpretation is that the inverse relation noted between sun- 

 shine and stem growth exists also between stem growth :.nd evapo- 

 ration. The question as to whether this relation ia an acci- 

 dental one due to the correspondence between the r:easonal 

 m rches of sunshine and evaporation or whether evaporation and 

 not sunshine is the determining factor here, is rather definitely 



