INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 301 



wounding, by the object that caused the wound. 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 wound- 

 ing, 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 bendings is still 

 not understood.'" 



Fig. 149. — Witches' broom on leaf of Pteris quadriaurüa, 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). 1 



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 fungus 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 Wurzelkrümmungen auf Entstehung und Anordnung 

 der Seitenwurzeln. Landw. Jahrb. 20: 361-426. 1900. 



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

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



