262 



COMPENDIUM OF GENERAL BOTANY. 



Tissue-tension may be mentioned as the first result of unequal 

 growth. 



(a) Tissue-tension. 



To illustrate this phenomenon a longitudinal section may be 

 cut from the middle portion of a growing stem or petiole (L in 

 Fig, 170, a). A vertical surface view of this section is shown in b 

 and c. The medulla m and cortex r do not cease to grow at the 

 same time as the woody tissue h. In the uninjured organ this 

 difference must produce tensions ; the woody elements are passively 

 elongated and continually strive to contract ; the cortical and med- 

 ullary cells are hindered in their growth, are compressed, and con- 

 tinually strive to elongate. 



771 



772 



FIG. 170. 



Isolating the individual parts of the section verifies the above 

 statement by the shortening or elongating of the various elements. 

 For the same reason the bisected organ curves outward. The 

 medulla elongates more than the cortex (c). Increasing the turgor 

 by placing the section in water will further increase the curvature. 

 Corresponding phenomena may be observed in transverse sections. 



From what has just been stated it follows that tissue-tensions 

 are produced by a decrease and increase in the turgor 1 as well as by 

 unequal growth. The following remarks will have a bearing upon 

 tissue-tension due to turgor. The form of the cells and their ex- 

 pansibility in different directions influences the phenomenon of ten- 

 sion in a high degree. In those roots which become shortened in 

 the turgescent state and elongated in the wilted state, we must 



1 According to N. J. C. MULLEB, a hydrostatic pressure of 13^ atmospheres 

 can be demonstrated in the medullary cells of Helianthm ; according to AM- 

 BRONN, 9-12 atmospheres in petioles of Fceniculum. 



