PLANT HUTKITION. 31 



The first icsult of this assimilation, chemists tell us, is 

 the formation of a soluble substance, " glucose," allied 

 both to starch and to sugar, and which, or a portion of 

 which, becomes starch, and is stored up for future use in 

 that form. No starch is formed in an atmosphere pur- 

 posely deprived of carbonic acid by the experimenter, 

 even if the cell be exposed to the light. Moreover, any 

 starch that may have been previously formed disappears 

 under such circumstances, just as it would do in dark- 

 ness, where the plant is dependent on its reserve stores 

 for its nourishment, and not on those which it procures 

 directly for itself when exposed to light in an atmosphere 

 in which carbonic acid gas forms a part. The changes 

 in question are presumed to take place, not in the pro- 

 toplasm itself, but in the chlorophyll grains ; at any rate, 

 it is in them that the starch first makes its appearance. 

 It is certain, also, that only cells which contain chlor- 

 ophyll and then only when exposed to light can 

 directly assimilate carbon. Cells without chlorophyll, 

 such as those of fungi, obtain their carbon by more indi- 

 rect and complex means. The vital importance of the 

 exposure of the leaves to sunlight might be inferred from 

 the bending of the stems and branches to the light, and 

 placing of the mobile leaves at such an angle as to receive 

 the full benefit of the sun's rays matters which will be 

 spoken of further on. 



What the Leaves do in Darkness. Inhalation of 

 Oxygen. In darkness (as well as under the influence of 

 light, in the case of those cells that do not contain chlo- 

 rophyll) changes go on of a different character to those 

 just described. There is, in fact, a constant elimination 

 of carbonic acid gas, and a corresponding absorption and 

 retention of oxygen gas. The interchange of these gases 

 has been compared to the corresponding changes in the 

 case of the respiration of animals ; but doubts have been 



