REMARKS ON THE GROWTH OF PLANTS. 
325 
There are many instances of the swelling membrane of plants in¬ 
creasing much faster downwards than in any other direction. A 
wounded stem or branch of an exogenous tree is healed quicker by the 
living membrane descending from the upper than it does from the lower 
side. If a wound be made on the stem of an herbaceous plant, as is 
performed in layering, the point of the dissevered part, called “ the 
tongue,” usually swells more than the other parts. The same kind of 
protuberance is often seen at the lower points of both grafts and inserted 
buds, and we may add cuttings also. When the new method of in¬ 
arching is performed—that is, by separating the graft entirely from the 
mother plant, and attaching it by its middle to the stock, and placing 
the lower end of the graft in a phial of water made fast thereto—the 
bottom so immersed very soon swells, and emits roots in the same way 
as a cutting placed in the ground. 
Now it would appear that all such instances of the protrusion down¬ 
wards of the actively increasing membrane of trees and shrubs, the 
downward lengthening of the cucumber, &c., are accomplished by 
something like gravitation, were it not that practical experiments care¬ 
fully performed show us that in these processes gravitation is not an 
agent. If the stem of a young tree in a pot be encircled by a tight 
ligature, or have a ring of bark taken off all round the same, and the 
plant and pot be reversed upon a lofty stage-top downwards—the root 
being duly supplied with water—the plant will grow, its shoots of the 
head turning upward, and the stem will swell unequally; the part 
above the band or incision—that is, the side nearest the hanging head 
of the plant—will be largest, and in the same way as if the plant and 
pot had been set upright. This experiment shows decidedly that it is 
not from any sinking of the sap that young roots—the living mem¬ 
brane—or other downward processes of a plant, progress with more 
celerity than the same do in any other direction. 
We have other instances of the unequal growth of vegetable mem¬ 
brane exemplified in the following particulars :—the most shaded side 
of the stem of a tree, and the lower side of the horizontal branches, are 
always farther extended from the pith, or centre, than the sunny side: 
if a tuber—a potato, for instance—be partly above ground, the exposed 
side swells much slower than the buried portion. 
We have already alluded to the circumstance of a wound on the stem 
of a tree being healed sooner by a new bark and wood which descends, 
than by the same which approaches from the bottom or sides of a scar. 
It was once a part of our duty to attend to the healing of a wound on 
the stem of a favourite tree, which required a period of eighteen years 
before a complete cure was effected. A very fine specimen of the 
