3] EFFECT OF EXTERNAL AGENTS UPON GROWTH 479 



cation of the protoplasm depending upon its very structure. 

 This general effect may be of two sorts : a grosser mechanical 

 one and a more refined. The grosser general effect is seen in 

 the osmotic action of dense solutions ; in the elongation of an 

 organ consequent upon pulling it with a considerable force ; in 

 the interference with vital actions by poisons, by intense light, 

 or by high temperatures. The more refined general effect is due 

 to the acceleration of growth by the acceleration of the essen- 

 tial metabolic processes involved in growth ; hence come the 

 effects, within limits, of favorable foods, of electricity, of 

 radiant energy, and of warmth. We can explain the effect of 

 such refined agents by known chemical laws. 



The grosser and the finer effects pass over into each other at 

 certain intensities. Thus the acceleration of growth produced 

 by summer temperature passes over at a higher temperature 

 into the gross retardation due to coagulation. 



The specific effects, in contradistinction to the general, appear 

 only in certain organisms or their parts, and have no such evi- ' 

 dent and explicable relation to the cause as the general effects 

 have to their causes. Such is the increased growth resulting 

 from certain slight poisons, from the deformations of the stems 

 of certain plants, from a wound which leaves a defect to be 

 filled by growth. These effects cannot at present be accounted 

 for satisfactorily by known chemical processes ; they result 

 from peculiarities of the specific protoplasms which depend 

 largely upon the past history of each kind of protoplasm. 



The same agent may have at different intensities, at one time, 

 the specific ; at another, the general effect. Thus the slight pull- 

 ing of the stem of a seedling may induce the specific shortening 

 effect, whereas a stronger pull will cause a gross elongation. 



The gross general effect, the refined general effect, and the 

 specific effect form a series of results brought about by pro- 

 cesses which are quite intelligible in the first case, much more 

 complex in the second, and still further removed from our ken 

 in the last ; we might speak roughly of these effects as due to 

 physical, chemical, and vital processes respectively, without 

 meaning to imply a qualitative difference between these pro- 

 cesses. The most complex of these three processes may also 

 be designated as response to stimulus. 



