142 



BOTANY OF THE LIVING PLANT 



very slight organic framework containing water. The mechanical 

 effectiveness of the internal turgor of the cells, and the insufficiency 

 of mere partitioning of a young or succulent part is shown by com- 

 paring a crisp fresh leaf with one which has withered, or has been 

 plasmolysed. 



There is, however, another factor which increases the mechanical 

 effectiveness of succulent parts in the young state, viz. the mutual 

 tensions of tissues. If a fresh young stem of Sunflower or Elder, or 

 any extending part of an herbaceous plant, be slit longitudinally into 

 quarters, these take strong curves. The outer surface of each quarter 



becomes concave, the inner faces of each 

 quarter convex. The curves become more 

 marked after the cut stem has been 

 steeped in water. These curves show that 

 the relations of the tissues in the living 

 stem are not passive. (Fig. 101, 2a, 2b.) 

 That the phenomenon is one of turgor of 

 the cells may be shown by allowing the 

 slit stem to wither, or by plasmolysing it 

 with a salt solution, when the curves dis- 

 appear, and the parts become limp. On 

 the other hand, if the several tissues be 

 completely separated from a measured 

 length of a fresh stem, and be themselves 

 measured after separation, the column of 

 pith will be found to have elongated, and 

 the outer tissues to have contracted. To 

 bring them back to their original state 

 the pith would have to be compressed and the outer tissues stretched. 

 (Fig. 101, a.) This is in fact their condition in the growing 

 stem. The pith tends to elongate, but is held back by the outer and 

 firmer tissues, which are thus kept tense. The relation of the inner 

 and outer tissues is then analogous to that of the wall and protoplast 

 in the turgescent cell, and the mechanical effect is the same. // is thus 

 seen that the firmness of succulent stems is due in large degree not only to the 

 turgor of the individual cells, but also to the mutual tension of its tissues. 

 Similar relations hold for the tissues also of leaves and roots ; but 

 these need not be described in detail. 



FIG. 101. 



i, Shoot of Sunflower with pith 

 separated by a cork-borer from the 

 outer tissues, za, split stem of Dan- 

 delion. 26, after immersion in water. 

 (After Strasburger.) 



