350 Morphogenetic Factors 



in tendrils having no stimulus of contact, but in the one under traction 

 the walls of the fundamental tissue cells were markedly thicker than in 

 the other tendrils. 



Compression. Because of technical difficulties, not as many attempts 

 have been made to produce the opposite sort of mechanical stimulation, 

 compression in the lengthwise direction. 



Pennington (1910) hung weights on woody and herbaceous stems of 

 various sorts as they were growing in height but found no appreciable 

 effect on structure or mechanical strength. Himmel (1927) used more 



S 



0RA.K5 FRJ533DRK 



Fig. 16-4. Graph showing growth of Podophyllum petioles, in inches per hour, under 

 lengthwise pressure from various weights. Upper curve, control; lower, experiment. 

 Growth is markedly reduced by pressure. ( From Himmel. ) 



favorable material, the growing petioles of the large, umbrella-like leaves 

 of Podophyllum. On the apices of these petioles he hung weights which 

 were periodically increased as growth continued. He found that growth 

 rate in the weighted petioles was less than in the controls but that growth 

 in the former finally equalled that in the latter (Fig. 16-4). The rigidity 

 of the petioles was somewhat increased. 



Rasdorsky ( 1925 ) , working with sunflower and marigold, approached 

 the problem in another way. He held up the plant by a gently stretched 

 thread attached to the upper part of its stem and thus relieved it from 



