PHYSIOLOGY 



185 



Collenehyma. All such strongly thickened, inflexible skeletal 

 elements have lost their capacity for . 



growth, and cannot, therefore, be _ M ^___^ ^ ^ / 

 utilised in those parts of plants which 

 are in an actively growing state. In 

 such cases when further support is re- 

 quired than is afforded by turgidity and 

 tissue tension, it is secured by the de- 

 velopment of COLLENCHYMA (p. 68). In 

 spite of the amount of water it contains, 

 this tissue, according to AMBRONN'S re 

 searches, opposes considerable resistance 

 to tearing, while it is easily stretched 

 by a longitudinal pull. When the part 

 of the plant containing it is growing more 

 or less actively, the collenchyma thus 

 readily accommodates itself to the 

 change in form. It forms, so to speak, the cartilaginous tissue of 

 plants ( 6 ). 



Fio. 179. Diagrammatic transverse sec- 

 tions, to show method of securing 

 rigidity by folding and rolling. In 

 the outspread organ A, to withstand 

 a pressure acting in the direction of 

 the aiTow, only the thickness d is 

 available, but in the folded organ B 

 the thickness D', and in the rolled 

 organ <' the thickness D". 



Fio. ISO. Buttress roots of a species of MC/VM//'". (After HAHERI.AXDT: from SCHIMPER'S 

 Pla/nt-Geography.) 



Since, as has already been pointed out, the resistance of the 



