DIRECT EQUILIBRATION. 437 



of appreciably changing that set o'' coiriplex rhythmical 

 actions and reactions constituting the life of the organism ; 

 and yet must not usually produce perturbations that are 

 fatal. Let us see what are the limits to direct equilibra- 

 •^ion hence arising. 



§ 161. In plants, organs engaged in nutrition, and exposed 

 to variations in the amounts and proportions of matters and 

 forces utilized in nutrition, may be expected to undergo cor- 

 responding variations. AYe find evidence that they do this. 

 The " changes of habit " which are common in plants, when 

 taken to places unlike in climate or soil to those before in- 

 habited by them, are changes of parts in which the modified 

 external actions directly produce modified internal actions. 

 The characters of the stem and shoots as woody or succulent, 

 erect or procumbent ; of the leaves in respect of their sizes, 

 thicknesses, and textures ; of the roots in their degrees of 

 development and modes of growth ; are obviously in imme- 

 diate relation to the characters of the environment. A per- 

 manent difference ia the quantity of light or heat, afiects, day 

 after day, the processes going on in the leaves. Habitual 

 rain or drought, alters all the assimilative actions, and 

 appreciably influences the organs that carry them on. Some 

 particular substance, by its presence in the soil, gives new 

 quaKties to some of the tissues ; causing greater rigidity or 

 flexibility, and so afiecting the general aspect. Here, then, 

 we have, ia plants, changes tending to briug about in them, 

 modified arrangements of functions and structures, in equi- 

 librium with modified sets of external forces. 



But now let us turn to other classes of organs possessed by 

 plants — organs which are not at once affected in their actions 

 by the variations of incident forces. Take first the organs 

 of aefence. Many plants are shielded against animals that 

 would else devour them, by formidable thorns ; and others, 

 hke the nettle, by stinging hairs. These must be counted 

 among the appliances by which equilibrium is maintained 



