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PHYSIOLOGY. 
BOOK II. 
CHAPTER II. 
ELEMENTARY ORGANS. 
The general properties of the elementary organs are, elasticity, 
extensibility, co7itractibility, and permeability to fluids or gaseous 
matter. The first gives plants the power of bending to the 
breeze, and of swaying backwards and forwards without 
breaking. The second enables them to develope with great 
rapidity when it is necessary for them to do so, and also to 
give way to pressure without tearing. The third causes parts 
that have been overstrained to recover their natural dimen- 
sions when the straining power is removed, and it permits the 
mouths of wounded vessels to close up so as to prevent the 
loss of their contents. The fourth secures the free commu- 
nication of the fluids through every part of a plant which is 
not choked up with earthy matter. 
The special properties of the elementary organs must be 
considered separately. 
That of these the cellular tissue is the most important 
is apparent by its being the only one of the elementary organs 
which is uniformly present in plants ; and by its being the 
chief constituent of all those compound organs which are 
most essential to the preservation of species. 
It transmits fluids in all directions. In most cellular plants 
no other .tissue exists, and yet in them a circulation of sap 
takes place ; it constitutes the whole of the medullary rays, 
conveying the elaborated juices from the bark towards the 
centre of the stem ; all the parenchyma in which the sap is 
diffused upon entering the leaf, and by which it is exposed 
to evaporation, light, and atmospheric action, consists of cel- 
lular tissue ; much of the bark in which the descending cur- 
rent of the sap takes place is also composed of it ; and in 
endogenous plants^ where no bark exists, there appears to be 
no other route that the descending sap can take, than through 
the cellular substance in which the vascular system is em- 
