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VEGETABLE PHYSIOLOGY. 
f igttolils ^.^{itisinlngii. 
By AETHUE HENFEET, Esq., F.L.S., LEcmtEE, on Botany at St. George's Hospitai,. 
THE ELEMENTAEY STEUCTUEE OF PLANTS. 
LEAVING now the general physical laws wliicli influence the processes of vegetahle hfe, we will pass 
to the examination of the structui-es of which plants are composed. It is necessary that we should 
enter a little here into the minute microscopic anatomy of plants, because the various portions of their 
construction are formed out of such a few simple elementary forms, and upon such a general plan, that 
the office which any organ or distinct part of a vegetahle fulfils, depends much more upon the condi- 
tion of its elementary structui-e than upon its form, or the position in which it is placed. For 
example, the stem, in its natiu-al condition, produces wood in layers giving thickness to it ; hut when 
parts of the stem are separated fi'om the main trunk hy which they are connected with the roots, they 
will usually, under favoui'ahle cii'cumstances, produce new roots from these same parts which otherwise 
develope woody layers ; again, in the generality of plants, the leaves are rightly regarded as the chief 
organs of the respiration, but in such plants as the Cactus, where the leaves are represented by mere 
dry spines, the surface of the stem takes upon itself the office of the leaves. This capability of adapta- 
tion depends on the simplicity of the organization, or the fact that every part of the plant is built up of 
collections of one and the same kind of elementary strnctui'e, vai'iously modified in their fully formed 
condition, according to the purpose to which they are destined. 
This word organ, perhaps, requires a little explanation, especially as it is applied to two classes of 
structure. In the first place, it is applied to the parts of plants, which, by their completeness of form 
and independence of habit, show themselves to be, to a certain extent, separate individual members of 
a collective whole ; thus the stem is one organ, the leaves are all separate organs, and so on. Again, 
when these are examined microscopically, they are found to be built up of separate and complete parts 
having a detinite form and constitution ; thus the spongy, succulent parts of all vegetable textui'es are 
built up of cells, that is, of little membranous vesicles or bladders, each one forming hy itself a closed 
bag, enclosing its own peculiar proportion and kind of contents ; these httle sacs are what we mean 
by elementary organs, the simplest form of the living structm-e, in the furthest state of division to 
which it can he carried without destroying its character as part of a living object. It is on this peculiarity 
of the constitution of the textures of living creatures, that the conti'ast is founded between the organic 
and the inorganic, or mineral creation, since in the latter all parts of the mass of substances are com- 
posed of material in a condition devoid of especial form, and capable of being divided as far as our in- 
sti'uments will allow us to carrj' the operation, and yet each particle mU retain all the characteristics 
(except the magnitude) of the entu-e mass ; in organic structures, on the contrary, forming vegetable 
and animal bodies, if we divide the elementary organs into pai'ts we at once destroy their pecuhar 
vital characters, and render them indistinguishable by any other means than that applied to mineral 
substances, namely, chemical analysis. 
All vegetable structures consist, in their earUest condition, of collections of membranous cells, as- 
sembled together in various ways and of vai'ious forms ; and it is only in a later stage of growth that 
these undergo changes which fit them to form part of the fii-m and solid portions, such as the woody 
substance of the stems and roots, therefore we will examine this simplest condition of the vegetable 
tissues first. 
If we make a very thin slice of such a soft, spongy part of a plant as the young pith, for instance, 
of the elder, and examine this by the microscope, we find it to be made up of vast numbers of little 
globular cells, formed of very deUcate membrane ; and as these, fi'om their shape, can only touch one 
another at certain points, lying in contact like a num- 
ber of cannon balls piled together, they afford us a 
good opportunity of seeing the independent, complete 
condition of these little elementary organs. In the 
young stages of the pith they are filled with hquid 
and solid matters, consisting of the substances which 
have been received into the plant to noui-ish it, and 
which undergo in such cells the chemical changes 
that give rise to the various properties of the sap. 
These contents of the cells must be left out of view 
for the present, until the various forms of the tissue 
In a thin slice of a leaf, in hlie manner, we find that tlig spongy substance, com- 
Fig. \. 
to show the cells 
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