202 GARDENING AS A SCIENCE. 
ascend through the cellular substance of the alburnum, and pass through vessels cellu- 
lar in structure which surround the bundles of the spiral tubes ; that the nutriment 
absorbed, becomes the true sap and living blood of the plant by exposure to light in 
the leaf, and that it descends by the bark, — wherever plants have bark — by which 
the matter which forms the layer of alburnum is deposited ; and that whatever 
portion of the true sap is not thus expended, sinks into the alburnum through the 
mis-named medullary processes, and joins the ascending current. As autumn, 
however, approaches, the expenditure of sap diminishes, and it then accumulates 
in the alburnum to be employed in forming the young shoots and leaves of the 
ensuing spring. I am in possession of a thousand facts to support this hypothesis, 
and not in possession of one in opposition to it." Sept. 2. 1830. 
Great and high as are our authorities, we must not attach to them a too im- 
plicit faith. Plants are living beings, or they are not ; if endowed with a vital 
principle, we cannot by any possibility prove that they are not more or less sentient, 
and, in a degree, however low it may be, possessed of the faculty of volition, and 
may, to a corresponding extent, select their food, and be susceptible of enjoyment. 
If, on the contrary, their vitality be purely vegetative, the only agent to which we 
can possibly refer the phenomena of the sap, its ascent, distribution, and assimi- 
lation, is electricity ; — and in truth, all the late discoveries of the decomposition 
of water, tend to confirm the hypothesis. This was candidly stated to Mr. Knight 
about the period when he became interested in the experiments of Dutrochet, 
which, as he justly observed, were productive of phenomena altogether wonderful. 
These phenomena, however, cannot bring to light the agency which induces the 
introduction of the living sap ; for, they only go to prove, — what Dr. Mitchell of 
America has subsequently confirmed, — that when membrane is interposed between 
fluids of different specific gravities, it operates as a medium of interfiltration 
between the two. So far, however, as membrane is concerned, the cellular mem- 
branous tissue of the alburnum and bark, containing fluids of various densities, 
may produce a variety of those attractive interblendings or organic changes, which 
have been too rashly termed chemical, since chemistry is utterly incapable to effect 
or to interpret one of the combinations which result from vital agency. 
On the question of electricity Mr. Knight thus expressed his opinions : — " I 
have long believed the electric fluid to be the agent that causes all secretions, 
properly so called, to take place both in plants and animals. The galvanic fluid 
will cause the process of digestion to go on, and of course the secretion of the 
gastric fluid. 5 ' " I thought, and still think, the discovery of these and other 
facts respecting the powers of the galvanic fluid of much importance in a medical 
point of view. I do not think that it can be applied in any way with advantage 
to plants. I witnessed the result of some experiments made under the inspection 
of Dr. Wilson Philip, and by M. Dutrochet, during a visit he paid me at Downton, 
and the effects were always injurious, however feeble the power. Nature gives 
enough, I conclude. " 
