798 MECHANICS OF GROWTH. 



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 the section 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 

 graduated in millimetres. 



