STIMULUS AXD RESPONSE 5 



habitat and the functional responses to them are considered 

 under adjustment. Growth is also placed here both for con- 

 venience and for the reason that it leads logically to the study 

 of modifications. In consequence, the treatment of adaotation is 

 practically confined to modifications of structure. 



6. Kinds of adjustment. With respect to the factor con- 

 cerned, the functional responses of the plant are distinguished 

 primarily as adjustment to water, light, or temperature. Re- 

 sponses to soluble salts are properly considered under water, while 

 the direct changes due to wind usually affect the form of the 

 plant alone. The response to gra\dty is so universal, final, and 

 absolute that it hardly falls within adjustment proper. Indi- 

 rect factors, i.e., such as soil, wind, rainfall, pressure, and physi- 

 ography, which can affect a function only by acting upon another 

 or direct factor, do not properly produce response, but the change 

 resulting from their influence is to be ascribed to the direct factor 

 concerned. For example, the effects of soil, physiography, wind, 

 and pressure are chiefly to be sought under adjustment to water, 

 because of their action upon water content or upon humidity. 



7. Normal and abnormal adjustment. The unusual stimuli 

 resulting from a greatly changed habitat or from a new one pro- 

 duce an unusual or abnormal response in function and often 

 in form. Adjustment is consequently to be regarded as normal 

 or abnormal. Normal adjustment is characteristic of a plant 

 that passes from youth to maturity in its owm habitat. The 

 functions are carried on in the manner usual to the species, and 

 there is in consequence no modification of structure. Abnormal 

 adjustment occurs in those plants that migrate into a new or 

 different habitat, or those whose habitat is seriously changed. 

 It is characterized, as a rule, by profound disturbance of function, 

 though the latter clearly depends upon the intensity of the change. 

 The most familiar cases of abnormal response are due to biotic 

 factors, particularh' parasitic fungi and insects. In most in- 

 stances of this sort, the disturbance is merely functional, but 

 often also the change in function is followed by a modification 

 in growth or structure, as in the "cedar apples" and "witches, 

 brooms" produced by rusts, and in the galls due to insects. 



A plant acted upon by a parasitic fungus or in.sect is said to 

 be in a pathological condition. The study of the effect of the 

 parasite upon the host-plant is called pathology, and it is regarded 



