348 Morpho genetic Factors 



that the plant would tend to react in such a way as to oppose the effect 

 of the factor and, in the cases here discussed, to develop more mechanical 

 tissue which would resist traction and prevent breakage of the plant. 

 Vochting (1878), with his versatile interest in all such problems, asked 

 the practical question as to whether plant axes, subject to different ten- 

 sile strains in nature, showed structural differences as a result. He com- 

 pared the pedicels of squash fruits hanging from trellises with similar 

 ones growing on the ground and found more vascular tissue in the 

 former. However, when such a fruit was supported on a platform beside 



Fig. 16-2. Effect of tension on wood structure. Left, transverse section of wood of 

 Fagus sylvatica from a root grown under strong tension; right, section of root of the 

 same species not under tension. Note generally thicker cell walls in the former. (From 

 Jaccard. ) 



a freely hanging one, he found the same amount of vascular tissue in 

 the stalks of both. Vochting suggested that the differences first observed 

 were due to differences in the amount of transpiration rather than to 

 tension. He also tried to induce mechanical tissue by traction in weak, 

 poorly vascularized plants, but without success. However, he grafted a 

 normal shoot on such a weak one and observed the development in the 

 latter of a marked increase of vascular tissue. 



It might be objected that stems are not usually subject to tension but 

 that roots are, and it was evidently important to study these organs as 

 well. Wildt (1906) fastened the seedling stem and the adjacent part of 



