440 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 
