231.1 BRIEF OUTLIXE OF VEGETABLE PHYSIOLOGY 



substances like starch and cellulose are formed ; (2) it is the solvent 

 in which all the vital chemical changes, like assimilation, are carried 

 on ; (3) its presence is an important factor in preserving the rigidity 

 of the plant body. The first of these offices has been touched upon 

 in the brief statement of assimilation made in the chapter on the 

 Leaf. The second need not be further dwelt upon. The third may 

 now be move fully considered, since it concerns a first essential to the 

 existence of the plant, namely: — 



534. The stability of the plant body. — By stability is meant the 

 power of the plant to keep its form, — the power, if it is an erect 

 plant, o£ keeping itself erect and outspread in proper position in all 

 its parts. It is a matter of common observation that plants snifering 

 from drought wilt and droop, sometimes even fall flat to the ground. 

 Wilted plants have partly or wholly lost their staliility. 



535. stability is secured in part by the properties of the tissues 

 themselves; the thick-walled, strengthening fibers are so disposed in 

 the stem as to secure the greatest rigidity. But in herliaceous and 

 succulent organs, firmness depends oftentimes as miub, or more, upon 

 the condition of the living cells in regard to their supply of water. 

 When one of these cells has a full supply of water, the expansive sub- 

 stances held in solution by the cell sap (for example, sugar and acids) 

 are enatded to distend the cell to its full limits.' The cell is then said 

 to be Inrqid. 



In such a condition it resists the distorting stresses brouglit upon it 

 by the pulls of neighboring cells. And when all the cells of a tissue 

 are fully turgid, they resist, collectively, all distorting stresses. Th.at 

 member of the plant body which is well watered, therefore, retains its 

 form and propier attitude. 



536. The turgidity of cellular tissues gives rise to tensions between 

 different masses of tissue lying side by side in the plant body. A good 

 illustration of these tissue tensions is furnislied by the succulent stalk 

 of a Pihubarb leaf. Let a poitiou of the fresh stalk be cut squarely 



1 Dissolved substances liave an expansive force, comparable in a gen- 

 eral way to the expansive force of gases. iSugar dissolved in cell sap presses 

 against the protoplasm that holds it in, just as hydrogen pre.sses against 

 the walls of a balloon. The cell, in such a case, has a constant tendency 

 to expand. If water is at hand that can come in to occupy the additional 

 space to be made by expansion, then the cell expands until the oiitward 

 push of the Sdlutious eijuals tlie resistance of the cell wall to lieing stretched. 

 The entrance of water, therefore, is tlie result i<i the expansive tendency of 

 the cell sap solutions. Water does not cause the swelling, only allows it. 

 Absorption of water by such action is called osmotic absorption. 



For a clear statement of the theory of osmotic pressure, see Ostwald's 

 '■Solutions," EuL'. trans. The theory, however, has received important 

 additions since the wurk named was pirblished. 



