INFLUENCE OF EXTERNAL CONDITIONS ON^ GROWTH 



271 



that object were still present as when the wound was made. This first response 

 is so slight that it is to be demonstrated only by very precise observation. 

 From forty-five to one hundred thirty-five minutes after the occurrence 

 of the wounding the second response begins, a bending in the lower portion of 

 the growing region. This second bending of the root is in the direction opposite 

 to that of the first, thus moving the root-tip as if to withdraw it from the 

 wounding object. The second bending is more pronounced than the first and 

 is of course the one studied by Darwin. The detailed mechanics of these bend- 

 ings is still not understood.™ 



Fig. 149.- 



-Witches" broom on leaf of Pteris qaadriaurita, caused by the fungus Taphrina 

 laurentia. {After Goebel.) 



Under usual conditions the laterals are distributed evenly over the surface 

 of the primary root, but when bends occur in the primary roots the secondary 

 ones develop in each bent region only on the convex side (Fig. 148).^ 



Parasitic fungi often cause striking changes in plant form and structure. 

 Sempervivum hirtum normally bears obovate leaves, about twice as long as 

 broad. When infected with the fungug Endophyllum sempervivi, however, this 

 plant produces leaves that are as much as seven times as long as broad. On 

 various trees and shrubs frequently occur peculiar structures known as "witches' 



1 Noll, F., Ueber den bestimmenden Einfluss von Wurzelkrummungen auf Entstehung und Anordnung 

 der Seitenwurzeln. Landw. Jahrb. 29: 361-426. 1900. 



" In connection with these traumatropic responses (or wound reactions), see: Spalding, 

 Vohiey M., The traumatic curvature of roots. Ann. bot. 8: 423-451. 1894. — Ed. 



