ix Leaves 163 



grind them to powder, thoroughly dry them at a high tempera- 

 ture, and weigh them. In this way the proportion of water 

 in the seeds may be estimated, and by simple calculation it 

 is easy to make an approximate statement as to the amount 

 of solid, and for the most part organic, matter in the seeds 

 used for the experiment. By and by, in favourable con- 

 ditions of light and heat, the plants (let us say of maize) 

 grow up and bear leaves. Let them flourish for several 

 weeks, and then remove them from the vessel in which 

 they have been growing ; dry off superficial moisture, cut 

 them into small pieces, and thoroughly dry a fair sample of 

 these at a high temperature sufficient to drive off the 

 internal water. It is thus easy to estimate how much 

 new organic matter the plant has made for itself during these 

 weeks. We say organic matter (starch and the like), 

 because the proportion of inorganic matter, estimated by 

 weighing the ash left after the whole is burned, is very 

 small, and may for the present be left out of account. 

 Now the experiment shows that in the course of its growth 

 the plant has gained greatly in organic matter. Where 

 has that come from? Partly, no doubt, from the water 

 and salts ; but that can be measured, and does not account 

 for the total increase of weight, nor do the water and salts 

 account at all for the carbon, which is the essential element 

 in all organic compounds. The answer to our question 

 every one knows ; the plant has absorbed carbonic acid 

 gas from the air, and has built the carbon thus obtained 

 along with the elements of water, and in part also with the 

 elements of salts, into complex organic substances. But by 

 what means was the carbonic acid gas absorbed from the 

 air? If we look at our maize plant there can hardly be 

 but one answer by means of the leaves. But if one of 

 the vessels containing germinated maize is placed in dark- 

 ness and allowed to grow there for some time, the resulting 



