MOTION OF THE SAP THROUGH PLANTS. 515 



stance rich in carbon, and that the process of becoming green was an active 

 deoxidation, or fixing of the carbon. The first who investigated this sub- 

 ject was Mulder; he at once came to the conclusion that chlorophyll was 

 a substance analogous to indigo, was rich in nitrogen, and that the process 

 of greening depends on an oxidating influence. He also discovered that 

 the changes of sugar, gum, starch, &c. into wax and fatty matters in 

 the herbaceous parts of plants, furnish the oxygen for this oxidation. 

 We find, therefore, chlorophyll combined with fatty and waxy matters. 

 It is equally well-known, since De Saussure's experiments, that the 

 fixing of the carbonic acid and the excretion of the oxygen in the plant 

 are dependant on the influence of light. 



I will call attention here to one point more, even at the hazard of 

 putting weapons into the hands of enthusiasts about vital power, as I am 

 persuaded that chemistry will not long leave the question unexplained. 

 Throughout the vegetable world we find the development of colours 

 depending on the action of light, and still, with few exceptions (perhaps 

 indigo and some resinous colouring matters), the colouring matter of plants, 

 after having been once developed, fades on being continuously exposed 

 to strong light, especially if the colouring matter be separated from the 

 entire plant. Thus chlorophyll and many of the colouring matters most 

 intense in colour, chiefly reds and blues, instantly fade on being exposed 

 to the light, as soon as they are separated from the plant. 



Of the influence of electricity I shall say nothing, because as yet we 

 know nothing. 



V. Motion of the Sap through Plants. 



204. All plants, from mosses upwards, distribute the absorbed 

 fluid, by endosmosis from cell to cell, through the whole plant. 

 Where there is the greatest evaporation, there is the greatest con- 

 centration of sap ; where there is the greatest activity, through 

 perhaps the change of thinner into thicker matter, there is the 

 greatest endosmotic power. Hence the greatest stream of sap is 

 directed to the green parts and the buds. This distribution or 

 absorption is uniform in all tropical plants which vegetate con- 

 tinuously. With plants of other climates it varies periodically, 

 according to the season. A point of time at length occurs when, 

 in consequence of meteorological changes, the chemical activity and 

 evaporation, as also the distribution and absorption of fluids, is 

 almost entirely suspended. On the approach of a genial season 

 they are again active. In what way the chemical activity, exhala- 

 tion, and consequently absorption, is put in motion, in the torrid 

 zone at the approach of rain, in the temperate zone at the approach 

 of spring, is yet unknown to us. Yet in the temperate zone heat, 

 and in the torrid zone moisture, appear to have the greatest share 

 in the process. We must conclude, therefore, that they are the 

 two principal conditions of the chemical processes. Even the 

 phenomena of their renewal of vital activity are only known to us 

 superficially. We only know thus much, that a great quantity 

 of fluid is drawn up with great power, that the starch already 



I. I. 2 



