OUTLINES OF THE PHYSIOLOGY OF NUTRITION 211 



should find little or no oxygen given off unless the 

 temperature of the water was at least 6 C. The 

 minimum temperature varies .very much for different 

 plants, but for every plant there is a certain 

 temperature below which no carbon dioxide can be 

 decomposed. 



Light is not only necessary in order that the 

 chlorophyll-containing organs may perform the work 

 of assimilation, but also for the formation of chloro- 

 phyll in the first instance. Although the protoplasmic 

 plastids are always present, the pigment chlorophyll 

 is not formed in them in the absence of light. If 

 a Potato plant be grown in the dark, it remains of 

 a sickly yellowish colour. This colour is due to a 

 substance called etiolin, which is distinct from chloro- 

 phyll, and may become converted into it when exposed 

 to the light. We must be careful to distinguish 

 between these two actions of light, which are quite 

 distinct : a plant will become green in the light even 

 though it be kept in air containing no carbon dioxide, 

 so we see that assimilation is not necessary for the 

 production of chlorophyll, though the presence of 

 chlorophyll is one of the necessary conditions of 

 assimilation. 



A green plant kept in the dark cannot decompose 

 carbon dioxide, and therefore under these circumstances 

 it forms no new organic substance. A potato kept 

 in a warm and damp but dark cellar will often grow. 

 It produces a pale, unhealthy plant, with long stems 

 and small leaves. If such a plant be dried and 

 weighed, we find that its solid substance is not 



