8i 
GARDENING AS A SCIENCE. 
VEGETABLE PHYSIOLOGY No. IV. 
Having investigated the external figure, or, as v;^e may term it, the skeleton^ 
of a plant to an extent, at least, which may stimulate the inquiring mind to 
investigate more profoundly, we are naturally led to its clothing or garniture. 
The Leaves — " those great sources of the permanent beauty of vegetation" — 
will be found, if we admit the theory of the late Sir H. Davy, ''though infinitely 
diversified in their forms, in all cases similar in interior organisation, and 
performing the same functions." Of these, he adds, " the green membranous 
substance may be considered as an extension of the parenchyma," ( cellular 
tissue^) "and the fine and thin covering as an epidermis" ( Agr. Chem.J. 
Lindley distinctly asserts that "a leaf is an expansion of the bark immediately 
below the origin of a regular leaf-bud, and is an appendage of the axis." And this 
opinion receives confirmation so far, that, in separating by natural exfoliation, a 
scar is left, which does not appear to extend below the bark : but then we find 
evident proof that there have existed connecting vessels and fibrous tissue ; and 
which, we suspect, have penetrated far deeper into the cellular substance of the wood 
of trees and shrubs, and into parts corresponding therewith in herbaceous subjects. 
Dr. John Aikin appears to sanction the idea that each leaf sends forth absorb- 
ing sap-vessels, which connect it with the trunk, and thus become its own 
individual supporters ; he thus expresses himself concerning the development of 
buds : — ^" The absorbent vessels of the leaf having exhausted themselves in the 
formation of the bud, are unable to bring it nearer to maturity ; in this state it 
exactly resembles a seed, containing within it the rudiment of vegetation, but desti- 
tute of absorbent vessels to nourish and evolve the embryo. Being, however, 
surrounded by sap, like a seed in moist earth, it is in a proper situation for 
growing ; the influence of the sun sets in motion the juices of the bud and of the 
seed, and the first operation in both of them is to send down roots a certain depth 
upon the inner bark of the tree, till they reach the part covered by the earth." 
The above theory was announced forty years ago ; and in what have our 
modern physiologists improved upon it ? It may not be true — nor, indeed, is it 
directly asserted — that each leaf sends a root to the ground ; but it teaches that 
everg hud is a system of life, for a time aided by the leaf which is usually situated 
immediately below it, (in the lime-tree — Tilia — the bud is actually ensheathed by 
the base of the footstalk,) but being embedded in juicy tissue, it is evolved in due 
season, and then sends forth radical fibres into the branch, while it propels 
upwards a new shoot with all its appendages. At this point we would again 
impress the important fact, that every new member of a plant is a development 
of something pre-formed ; and this fact becomes still more evident by the almost 
immediate effect of close pruning ; take, for example, a shrub which, as a semi- 
VOL. XI. NO. CXXIV. M 
