130 GAIN IN DRY WEIGHT [CH. 



it means that the seedlings in the dark have lost much of 

 their carbon by respiration, while those in full light have 

 gained so much carbon that it far more than covers what 

 has been lost in respiration : those in diffuse light, on 

 the other hand, while they have more carbon than the 

 plants in darkness, have gained far less than those fully 

 exposed to light. All three sets of plants have lost the 

 same proportion of carbon by respiration a process always 

 going on but the plants in the light have recovered 

 much more than they have lost, by means of carbon- 

 assimilation in their green leaves. 



Gain in dry weight i.e. increase of carbonaceous 

 substance is one of the essential features in this process, 

 and it now remains to prove more exactly that this in- 

 crease is really due to the activity of the leaves, and not 

 to that of the roots. 



It is well known that a seed may be germinated over 

 water, instead of being plunged in wet soil, and that so 

 long as the seedling is supplied with sufficient water, it 

 will develope into a young plant with root, stem and 

 leaves, at the expense of the reserve materials stored up 

 in the endosperm or cotyledons as the case may be ; but 

 that when these reserves are exhausted the plantlet dies 

 unless other supplies are given to it. 



Suppose, now, we repeat the foregoing experiment 

 with seedlings germinating with their roots in water to 

 which traces of the essential mineral matters of the soil 

 are added e.g. calcium nitrate, potassium nitrate, mag- 

 nesium sulphate, potassium phosphate, and a trace of 

 chloride of iron. It will be found that the same results 

 are obtained. The plants in full daylight increase in 

 carbonaceous dry weight, those in the dark decrease and 

 soon die, while those in diffuse light just manage to 

 keep going, for a time at least. 



