44 o PHYSIOLOGY 



increase in mechanical tissues, but flexure, 'with its alternating compres- 

 sion and tension, such as the wind in certain regions produces, beyond 

 doubt increases the proportion of mechanical tissues and thickens their 

 walls. 'When combined with excessive evaporation and perhaps other 

 unfavorable factors, the effect on bodily form is astonishing (see Part III 

 on stem-dwarfing). 



Deformities. Noteworthy local modifications of form are produced 

 by the attacks of parasites, either plant or animal. When specific 

 deformities are produced, the structures are called galls (fig. 655, p. 384). 

 Just how far these are due to chemical substances excreted by the para- 

 site, and how far to the mechanical pressure, to the punctures, or to the 

 movements of the larvae of animal parasites, remains at present quite 

 uncertain. Whether chemical or mechanical stimuli act upon the host, 

 its response might be first an altered metabolism, which produces ap- 

 propriate effects upon the division and course of development of the 

 cells, resulting in the deformation of the region. Profound alterations 

 in the relative development of the tissues and in the character of their 

 elements accompany the deformity. 



Injuries. Injuries of various sorts call forth growth in tissues which 

 have long passed the ordinary period of cell -division. This gives rise to 

 a callus at the edges of the wound which tends to close it, a fact that is of 

 great practical service in the grafting and budding so indispensable in 

 fruit growing. Desirable sorts, too tender for a given climate, may 

 thus be united with stocks that are hardy, but have no good qualities 

 in their fruit. 



In practice, smoothly cut surfaces are opposed and kept in close contact, with the 

 exclusion of water and spores by wrappings and wax. The healing tissues blend, 

 as they form at the junction, and an organic union is established, permitting the 

 passage of water and foods freely. 



If a wound be allowed to heal, the callus may give rise to new growing 

 points, from which the regeneration of removed organs may proceed. 

 Thus, if a root be decapitated, a new apex may be regenerated, if the 

 cut be near enough the tip, or new lateral roots may arise that would 

 not otherwise have been produced, or old roots may be incited to more 

 active growth. In either case of the formation of new organs, the reac- 

 tion to the wound stimulus is complicated with unknown factors named 

 polarity, and with the influence of other organs called correlations. 



Polarity. Since the opposite ends of an egg cell give rise to unlike 

 structures (for example, in seed plants, suspensor cells from one end and 



