386 
VEGETABLE PHYSIOLOGY. 
already contains the necessary chemical principles, To this it is ob¬ 
jected that plants grown in water alone never arrive at perfection, or 
mature their seeds. But this is not strictly true ; they do perfect their 
seeds: but it is not surprising that crude water should be insufficient 
for purposes which are fully answered by water properly mixed and 
tempered.’ 
That the extractive matter contained in earth was the real food 
of plants "was long ago stated by Woodward and Kylbel; and most 
physiologists have adopted this opinion. But it has been estimated 
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. Now, supposing this calculation to be not far from the truth, 
it serves to show that extractive matter and carbonic acid are not alone 
sufficient for the nutriment of plants. 
“ Nevertheless, if neither extractive matter nor carbonic acid 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 by far the greater part of their food. It is well known 
that roots cannot perform their functions unless within the reach of the 
atmosphere. This arises from the necessity for their feeding upon car¬ 
bonic acid, which, after having been formed by the oxygen of the 
atmosphere combining with the carbon in the soil, is then received into 
the system of the plant, 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 ascertained that, 
feed plants as you will, they will neither grow nor live, whether you 
offer them oxygen, hydrogen, azote, or any other gaseous or fluid prin¬ 
ciple, unless carbonic acid be present. 
The course which is taken by the sap , after entering a 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 been long disproved by modern investigators, and especially 
by the experiments of Mr. Knight. If a trunk be 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. If a 
branch be cut through at the same season, it will be found that, while 
the lower face of the wound bleeds copiously, scarcely any fluid exudes 
from the upper face; from which and other facts it has been fully 
ascertained that the sap rises through the wood, and chiefly through 
the alburnum. Observations of the same nature have also proved that 
the sap descends through the liber. But the sap is also diffused late- 
