The Sap. 263 



poses which are fully answered by water properly mixed and 

 tempered. That the extractive matter contained in the earth was 

 the real food of plants was long ago stated ; and most physiolo- 

 gists have adopted this opinion. But it has been estimated by 

 Theodore de Saussure, that a plant, when dried, does not derive 

 more than a twentieth part of its weight from extractive matter 

 and carbonic acid dissolved in water ; supposing this calcula- 

 tion not very accurate, it is not at least very far from the truth, 

 and at least serves to show that extractive matter and carbonic 

 acid are not alone sufficient for the nutriment of plants. Never- 

 theless, if neither of these two can be considered to constitute 

 exclusively the food of plants, it is at least quite certain that they 

 not only cannot exist without the latter, but that it forms a prin- 

 cipal part of their food. It is well known that roots cannot per- 

 forin their functions unless within the reach of the atmosphere ; 

 which arises from the necessity for their feeding upon carbonic 

 acid, which, after having been formed by the oxygen of the at- 

 mosphere combining with the carbon of the soil, is then received 

 into the. system of the plan!, to be impelled upwards, dissolved in 

 the sap till it reaches the leaves, where it is decomposed by light, 

 the oxygen liberated and the carbon fixed. It has also been as- 

 certained that treat plants as you will, they will neither grow 

 nor live upon any element unless carbonic acid is present. 

 Those principles are called foreign to plants that cannot be refer- 

 red to the four elements we have mentioned, such as the various 

 salts. That some plants have the power of secreting one kind 

 of accessory principle, and others another kind, from the same 

 food, is clear, from the fact that if wheat and peas be grown in 

 the same water, earth, or medium, the former will uniformly de- 

 posit silex in their cutiele and the latter never 



The course which is taken by the sap, after entering the plant, 

 is the next subject of consideration. The opinion of the old 

 botanists was, that it ascended from the roots between the bark 

 and the wood ; but this has long been disproved by modern in- 

 vestigators, and especially by the experiments of Mr. Knight. 

 If a trunk is cut through in the spring, at the time the sap is 

 rising, this fluid will be found to exude more or less from all 

 parts of the surface of the section, except the hardest heart wood, 

 but most copiously from the alburnum. Observations of the 

 same nature have also proved that it descends through the liber. 



