76 THE PLANT SKELETON 



The Making of the Skeleton. The necessary strength 

 and hardness of the skeleton is obtained by modifications of the 

 cell-wall and changes in the forms of the cells. The ordinarily 

 thin walls do not suffice to hold the plant body erect by their 

 own strength, but when filled with cell sap until they are 

 stretched they become rigid, like a toy rubber balloon when 

 inflated with gas, and in that way hold the body firm and erect. 

 Plants or parts of plants that depend upon this condition for 

 strength soon wilt when they begin to lose water faster than 

 they take it in. 



The skeletal tissues make plants more or less independent 

 of fluctuations in the water supply in maintaining the right 

 form and position of the body. The modifications of the wall 

 involve thickening of the originally thin wall, chemical altera- 

 tions of the wall by changes in the old materials and deposi- 

 tions of new, and transformations in the physical condition 

 of the wall, such as its hardness and elasticity. The changes 

 in the form of the cells consist, as a rule, in their elongation 

 parallel with the line of action of the main forces which they 

 are to resist. While the cells are elongated their ends usually 

 glide past each other and form spliced joints which greatly 

 increase the strength of the tissue (Figs. 15 and 19). 



THE TISSUES OF THE SKELETON 



The Collenchyma. This is the first skeletal tissue formed. 

 It appears in stems a short distance below the growing apex 

 where the bast and wood have as yet not begun to be formed, 

 and it is therefore the only tissue thus far having a strength- 

 ening function chiefly. Its chief characteristic is that its walls 

 are thickened at the angles where three or four cells join, and 

 this thickening in extreme cases becomes so great as almost to 

 close the cell cavity. While the angles are thickening a median 

 strip of the wall is left thin, clearly in order to allow a flow of 

 sap from cell to cell. The walls, as a rule, remain cellulose 

 throughout (Figs, n and 14). 



