20 THE WORK OF THE LEAF [chap. 



mustard seeds. When they are well up and showing 

 green leaves, add one-tenth of a gramme of nitrate of 

 soda to the water you are giving to the pot, and let the 

 growth continue for a few weeks longer. Finally, when 

 the plants show signs of flowering pull them up, wash 

 away all traces of sand from their roots, put them in the 

 oven, and weigh them when dry. They will weigh eight 

 to ten or twenty times as much as the seed from which 

 they started. It is not even necessary to add to the pot 

 of sand the plant ash or the nitrate of soda — the weighed 

 seeds may be sown on a bed of sand or a strip of clean 

 flannel kept moist in a soup plate, the little plants being 

 taken off when they will grow no longer. In this case, 

 however, the increase of weight will not be so great. 

 Or, again, one of the water cultures to be described 

 later may be used to afford a comparison between the 

 weight of the seed and the weight of the plant arising 

 from it when it has not been in contact with any soil. 

 Now the point of all these experiments is that the 

 increased weight of the plant is largely made of carbon 

 (we have already seen that more than half of the dry 

 matter of a plant is carbon), yet neither the sand, the 

 plant ash, the nitrate of soda, nor the water used in the 

 first experiment, nor the solution used in the water 

 culture, contain any carbon at all. It is true that the 

 flannel we have also suggested as a medium on which 

 the mustard seeds could be grown is a compound of 

 carbon, but it is such a compound as the plant cannot 

 feed upon, and is as inert as so much sand. If, then, 

 the plant gains carbon as it grows, and there is none in 

 the water or the support on which it grows, where does 

 the carbon come from ? There remains only one source 

 — the air which surrounds the plant ; and later experi- 

 ments will make more evident the fact that plants do 

 derive their chief sustenance from the air. In these 



