SPECIFIC GROWTH FACTORS 339 



from the wounded side. This has been interpreted as purposeful, 

 but it is probably connected with the fact that wounding may 

 result in greater respiration and greater stimulation of activity, 

 which in turn produces greater growth on the wounded side, 

 thus bending the tip away. If the wound is severe, growth is 

 checked on the side of the wound and the bending takes place 

 towards the wound. This fact alone is sufficient to overthrow the 

 " adaptation" theory. 



When stung by insects the curious formations known as galls 

 occur. These differ in size and shape with the insect and the plant, 

 and are so characteristic that cecidology has become a complex 

 taxonomic science. Similar outgrowths may be produced by the 

 attacks of parasitic fungi, among which are those causing the 

 "witches'-brooms" found on ferns, hackberries, and ash. 



The type of growth after wounding is of interest in phylogeny, 

 and is thought by morphologists to throw light upon the ancestors 

 of the plant. Wounding causes a return to an ancestral condi- 

 tion or the formation of atavistic structures which may be of 

 extreme interest. Thus the flower heads of the hawk's-beard 

 (Crepis biennis) when infected with the mite, Eriophyes, become 

 transformed into umbel-like structures, which are interpreted as 

 an ancestral type. Similarly the dioecious plant, Melandryum. 

 album, has perfect flowers when attacked by the smut, Ustilago; 

 and the fir (Abies) produces resin canals when wounded. Such 

 changes are all thought to be reversions to an ancestral form. 



Mechanical Stimuli.— When pressure is applied to an organ, 

 growth is retarded. When a bean root was placed in a gypsum 

 cast for about a month, not only was enlargement checked but 

 spiral and pitted vessels, which in ordinary roots are not found 

 nearer than 3 cm. of the tip, were here found within 2 mm. A 

 cross section from such a confined root taken 2 mm. from the tip 

 has the same appearance as a section 30 mm. from the tip of a 

 normal root. The stimulus caused by contact may also deter- 

 mine the location of organs. Thus the location of roots or tendrils 

 on climbing plants is often determined by the places of contact be- 

 tween the climber and the support. Similarly the haustoria of the 

 parasitic dodder (Cuscuta) occur where the parasite and host 

 come in contact, and seem to be determined by the stimulus of 

 contact or pressure. 



Tension first retards the growth of the organ and then stimu- 



