MEASUREMENT OF TISSUE-TENSIONS. 



571 



parenchyma, may easily be demonstrated in growing petioles, especially those of 

 considerable length and thickness, such as those of the Beet, Rhubarb, Philo- 

 defidron, &c. 



If a growing internode or leaf-stalk is split by means of a longitudinal section, 

 or by two sections at right angles, the parts curve concave outwards, because the pith 

 side is extended and the epidermis contracts, a state of affairs which must necessarily 

 lead to such curvature. The phenomenon appears most clearly if a lamella is 

 taken from the middle of the whole organ by means of two parallel longitudinal 

 sections, and this lamella laid flat on the table and the pith halved lengthwise ; then, 

 in proportion as the knife travels forward, the two halves also curve concave outwards. 

 If now, instead of halving it, thin strips of tissue are separated from this median lamella 

 of the organ, progressively from without inwards — first one which contains the 

 epidermis, then one which contains the cortical tissue, and then a further one 

 containing the young as yet unlignified woody layer — they all curve concavely out- 

 wards; because the layers which abut on one another are all extended negatively 

 on the outer side and positively on the inner side, so that on separating them as 

 above the outer side of each strip must shorten and the inner side lengthen. Here 

 also I may adduce a few examples from my very numerous measurements. 



A considerable rapidity of growth in length, with a simultaneous progressive 

 differentiation of tissue-layers, as met with in upright leaf-shoots, strong petioles, 

 and tendrils, seems necessary to promote this tissue-tension; since it is not 

 observed in very slowly growing shoot-axes, and in these cases the tissue- 

 differentiation also proceeds but slowly, e.g. in the downward growing thick 

 rhizomes of Yucca and Draccena. Finally, no morphological differentiation of 

 tissues at all is necessary to produce such tension of tissues : this is proved by 

 the stalks of growing Agarics, which consist entirely of homogeneous hyphal 

 tissue, and nevertheless exhibit powerful tensions between the outer and inner 

 tissue-layers. 



When, on isolation, the previously passively extended tissues become suddenly 

 shortened, and the previously positively distended pith as suddenly lengthens, both 

 can only be effected by means of a corresponding change in form of the cells ; for 

 even a merely relatively large alteration in volume is not to be thought of, because 

 neither the water of the contents nor the membranes saturated with water can 



