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CHAPTER I. 



ELEMENTARY ORGANS. 



The general properties of the elementary organs are elasticity, 

 cxtensihility, contractibility, and permeabiliti/ to fluids or gaseous 

 matter. The first gives plants the power of bending to the 

 breeze, and of swaying backwards and forwards without break- 

 ing. 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 it permits the mouths of 

 wounded vessels to close up so as to prevent the loss of their 

 contents. The fourth secures the free commimication of the 

 fluids through eveiy 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 oi-gans 

 that is uniformly present in plants ; and by its being the chief 

 constituent of all tiiose compound organs that are most essen- 

 tial to the preservation of species. 



It transmits Jliiids 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, convey- 

 ing the elaborated juices from the bark towards the centre of 

 the stem; all the parenchyma in which the sap is difliised 

 upon entering the leaf, and by which it is exposed to evapor- 

 ation, light, and atmospheric action, consists of cellular tissue ; 

 nearly all the bark in which the descending current of the 

 sap takes place is also composed of it ; and in endogenous 

 plants, where no bark exists, there appeal's to be no other 



