VEGETABLE PHYSIOLOGY. 
f igttnlilE ^^luiBinlngtf, 
By ARTHUR HENFEEY, Esq., F.L.S., Lectuker on Botany at St. Geokge's Hospital. 
PHYSICS OP VEGETATION. 
fN accordance -with, the plan stated in the preceding paper, I shall iii'st dii-ect attention to the 
principal physical phenomena, which "we shall always have to bear in mind when inquii-ing into 
questions of vegetable physiology. 
These will be examined most satisfactorily if we take them in the order of the relative position of 
the different forces acting in nature. The first and simplest of these is the force of gravity, which, 
passing over its influence in asti'onomical questions, is exhibited upon the earth in the effect of what 
we call weight. All the particles of the matter composing the earth, and the various bodies upon its 
siu'face, are always subject to an attraction toward the centre of the earth; and, as the particles are all 
equally attracted, bodies will be more forcibly drawn toward the centre, in proportion to the number of 
pai'ticles of wliich they are composed. Thus a pound weight made of brass will be attracted more 
strongly to the centre than an ounce weight of the same metal, because the fu-st is composed of a 
greater number of particles ; being thus more strongly attracted, we say, in common language, it has 
more weight, or is heavier. But there is another point to be noticed of more importance to us in our 
present study, viz., the difference of weight arising fi-om difference of density. A bottle filled with 
sand is heavier than when it is filled mth water ; and the explanation of this is, that the sand is a 
denser substance than water, which means, that the particles in the sand being closer together than 
those of the water, there are a greater number of them contained in the same space. This difference 
of density produces what is called the specific gravity, or specific loeiyhi, of substances, which is the 
weight of equal bulks of bodies of different densities, in proportion to a fixed standard, such as that of 
the weight of a fixed bulk of water. Solids are more dense than Uquids, hquids than gases or au-. 
The practical conclusions to be drawn fi'om these statements are chiefly of importance in examining 
the mode of distribution of the nutrient fluids in plants, since we thus understand why any cause which 
increases the density of a fluid ^vill give it a tendency to descend in the structures. 
The general phenomena resulting fi-om the action of gravity are famihar to every one, and require 
but a few remarks. It may be worth while, however, to mention one or two points which strikingly 
illustrate the peculiar relation of vegetables to the physical force of gravity. Vegetables, as complete 
bodies, can only resist the force of gravity by mechanical means, and have (or in only a very few 
exceptional cases) no power of opposing a voluntary force to it, as animals do in their movements 
by means of their muscles. The rigidity, toughness, or mechanical strength of theii- structures, is the 
power wliich enables most plants to sustain weight, and we see this strikingly exhibited in trees, which 
are built up -nith the most beautiful mechanical perfection of arrangement. The pyramidal form of 
the trunk, with its spreading roots, in the Dicotyledonous tree, the pyramidal collection of roots succes- 
sively developed Hke so many slanting props or stays around the 
base of the tall slender Palm, are examples which at once present 
themselves to us. On the other hand, when the rigidity is wanting, 
the weight of the upper part of the plant either causes it to He pro- 
strate, or requii-es that it should be attached to some extraneous sup- 
port. In aquatic plants, which are generally of very dehcate stmcture, 
the mechanical adaptations are not less striking. Plants, so soft and 
weak that they faU into a confused entangled mass when lifted into 
the air, are buoyed up in water, as a much denser fluid than air ; 
partly, from the fact that the specific yrarity of then- structui-es is 
of very little more specific gravity than the water, and so they do not 
readily sink in it, and partly by the contrivance of swimming blad- 
ders, as it were, consisting of hoUow spaces fUled with air, which, 
tending to rise like bubbles in the water, float up the structm-es which 
inclose them. We find examples of the fu'st in all the more dehcate 
filamentous or membranous Algoe, and, of the latter, perhaps the most 
sti-iking instances are furnished by the Bladderwrack {Fuciis vesicido- 
siis) occm-ring in all oui' seacoasts, and by the Utricuhria, or Bladder- 
wort, the leaves of which are fm'nished with a number of little hoUow receptacles containing aii\ Air 
chambers, consisting of regular or u-regular closed cavities are also found almost universally on the 
and leaves of floating aquatic plants, sometimes exhibiting great regularity in then- arrangement. 
If 
EL/UJDERWHACK {FtlCUS VESICnLOSt'S). 
