FUNCTION.] CELLULAR TISSUE. 171 



CHAPTER IV. 



OF THE ELEMENTARY ORGANS. 



THE general properties of the elementary organs are, 

 elasticity, extensibility, contractibility, and permeability to 

 fluids or gaseous matter. The first gives plants the power of 

 bending to the breeze, and of swaying backwards and for- 

 wards 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 dimensions when the straining power is removed, and 

 permits the mouths of wounded vessels to close up so as to 

 prevent the loss of their contents. The fourth secures the 

 free communication 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. No other tissue is found in the embryo, through 

 which, nevertheless, fluids pass freely ; the spongioles, and all 

 other absorbents consist of nothing else ; 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 cellular tissue ; much of the 



