346 Morpho genetic Factors 



responses to contact, involve more definitely morphogenetic changes. 

 When the tip of a tendril is lightly touched on one side, as by a small 

 branch or wire, the tendril will coil around it and thus tend to anchor 

 the plant to a support. This coiling results from the more rapid growth of 

 that side of the tendril not touching the support. There is evidence that 

 the stimulus of contact tends to produce a substance that checks growth, 

 although the mechanism which makes a tendril react thus, and later 

 contract in a coil, pulling the plant toward a support, is not well under- 

 stood. 



Mechanical factors are also concerned in other growth reactions. Biin- 

 ning and his colleagues (1941, 1948, 1954), working with Mimosa, Si- 

 napis, and Vicia, have forlnd that mechanical stimulation (stroking with 

 paper or agitating on a shaking machine) checks the lengthening of 

 stems in darkness ( etiolation ) in much the same way that light does. The 

 internal structure of stems grown in darkness but mechanically stimu- 

 lated is quite different from that of etiolated stems and hardly to be 

 distinguished from ones growing in the light (Fig. 16-1). He suggests 

 that both mechanical stimulation and light partially inactivate auxin ac- 

 tion. Both stimuli are more effective if repeated at intervals than if applied 

 continuously, a fact which may be due to a refractive stage following 

 the stimulus. 



Borriss ( 1934£> ) showed that Coprinus fruiting bodies which would not 

 have matured in the darkness will do so, at least partially, if mechanically 

 stimulated, and Stief el ( 1952 ) finds that the stipe of the fruiting body of 

 Coprinus responds to mechanical stimulation and to light just as do the 

 stems of higher plants, both stimuli tending to check elongation. 



Mechanical pressure may have an important morphogenetic effect by 

 determining the plane of cell division in meristematic tissue (p. 49), the 

 plane of division tending to be parallel to the direction of the pressure. 



It is with the more specifically morphogenetic effects of mechanical 

 stimuli, however, that we are particularly concerned here. Chief among 

 these are tension, compression, bending, swaying, and the omnipresent 

 stimulus of gravity. 



Tension. Much work has been done on this problem, especially in 

 earlier years, but the results are often contradictory. Hegler (1893) 

 stretched seedlings of sunflower and petioles of Helleborus by attaching 

 weights to them. After a 2-day application of 150 gm., it required a 

 pull of 350 gm. to break these structures as compared with only 160 gm. 

 in the controls. Traction seemed to have increased the tensile strength, 

 and Hegler found that traction had increased the cell-wall thickness and 

 the amount of collenchyma. 



Newcombe (1895) reviewed the very considerable amount of early 

 literature in this field. His own work confirmed Hegler's. He also found 



