49" LECTURE XIX. 



Combined Effects. 



We pass now to consider the combined effect of the various 

 directive influences which, as we have seen, determine the 

 jiirection of growth of plant-organs, and which we have so far 

 studied individually. 



The ultimate position assumed by a plant-organ in the 

 course of its development and growth is a resultant one: it is 

 the resultant of the action of the inherent directive influences, 

 and of the external directive influences to which the organ is 

 sensitive. 



With regard to the inherent directive influences, we have 

 found them (pp. 421, 418) to be two: the relation of the 

 organs to each other, and their own rectipetality. The 

 former determines what the relative directions of growth of 

 the organs of a plant are and these directions they maintain 

 in the absence of any disturbing external stimulus. An 

 evident manifestation of this can, in fact, only be obtained 

 when the plant is grown under such conditions that its 

 organs are exposed as little as possible to the action of 

 external directive influences, as, for instance, when it is 

 grown in darkness and is rotating horizontally on the cli- 

 nostat. Let us consider the case of a plant growing under 

 these conditions. Assuming that the soil in which the root 

 is imbedded is uniformly moist, the only external directive 

 influence acting upon the primary ^hoot and the primary 

 root will be that of the substratum; but, inasmuch as this 

 is clearly not acting so as to cause somatptropic curvature, 

 its effect may be neglected. The primary shoot will con- 

 tinue to grow in a straight line horizontally outwards, and 

 the primary root to grow in a straight line horizontally in- 

 wards, in virtue of the opposition of the directions of growth 

 of these organs (p. 419). The lateral shoots will arise from 

 the primary shoot and the lateral roots from the primary 

 root at their own proper angles, and will continue to grow 

 in a straight line in the directions thus impressed upon them. 



