l7A BOTANY OF THE LIVING PLANT 



citable for a salad gives by weight 95 per cent, of water, 

 and only 5 per cent, of organic material for cell-walls and proto- 

 plasts Such a structure is in fact a very slight organic framework 

 containing water. I he mechanical effectiveness of the internal turgor 

 the cells, and the insufficiency of the mere partitioning of a young 

 or succulent part is shown by comparing 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 



ions 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. 108, 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. 108, I.) This is in fact their condition in the growing stem. 

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

 -, which are thus kept tense. The relation of the inner and 

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

 in the turgescenl i ell, and the mechanical effect is the same. // is thus 

 I 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 tensions of their 

 tes. Similar relations also hold for the tissues of leaves and roots, 

 but these need not be described in detail. 



Fie. io8. 



I, Shoot of Sunflower with pith 

 separated by a cork-borer from the 

 r tissues. 2a, split stem of Dan- 

 delion. 2b, after immersion in water. 

 • r Strasburger.) 



