PHENOMENA DUE TO THE TENSION OF TISSUES. 



7J9 



It is easy to establish the existence of similar contractions of the outer tissues 

 and elongations of the parenchyma in the case of growing leaf-stalks, as those of 

 Beta, Rheum, Philodendron, &c. 



If a growing internode or a leaf-stalk is split by two longitudinal sections at 

 right angles to one another, the parts will bend concavely outwards, evidently in 

 consequence of the lengthening of the pith and contraction of the outer tissue. This 

 phenomenon is seen most clearly if a thin longitudinal slice is taken from the middle 

 of the internode, laid flat, and the pith then halved lengthwise ; as the knife advances 

 the two halves will bend concavely outwards. If, instead of cutting it in two, thin 

 strips of tissue are cut proceeding from without inwards, first one including the 

 epidermis, next one including the cortical tissue, and finally one including the 

 wood, they will all bend concavely outwards, because the adjacent layers are all 

 in a state of negative tension on the outside, of positive tension on the inside, and 

 when separated, the outer side always becomes shorter, the inner side longer. 



That this bending is caused by simultaneous contraction of the outside and 

 lengthening of the inside, is at once clear from the measurements already given, but 

 may also be observed directly, as will be seen from the following table. Longi- 

 tudinal slices of considerable thickness were cut from the middle of growing inter- 

 nodes, laid flat, and the pith then halved by a longitudinal cut ; the radius of the 

 curvature which each half at once assumed was determined, and the length of the 

 convex inner and the concave outer side measured by means of a strip of card 

 oraduated in millimetres. 



Silphium perfoliaimn. 

 Left half 

 Right half 



Silphmm perfoliatwn. 

 Left half 

 Right half 



Macleya cor data. 

 Cavitv. 



Length of 

 the entire 

 internode. 



69-5 mm. 

 69*5 



190 

 190 



134-5 



Radius of 

 curvature of 

 the segment. 



4 cm. 

 4 



3—4 

 3—4 



.^—6 



Contrac- 

 tion of the 

 concave 

 outer side. 



2'8 p.c. 

 2-4 



2-8 

 2-6 



Lengthen- 

 ing of the 



convex 

 inner side. 



9-3 p.c. 

 93 



95 



IO-8 



71 



Semidiameter 



of the 



internode. 



3 mm. 

 3 



3'5 

 4-5 



33 



