70 PLAHT LIFE OK THE FARM. 



comes - into contact with the straw of a wheat plant, 

 growth is checked on the surface by which contact is 

 made, while it is increased on the opposite side. As a 

 consequence, one side of the climber is flattened against 

 the supporting plant, while the other side, growing more 

 rapidly, becomes convex, and its tip is forced in process 

 of growth round the supporting stem. The increased 

 growth on the convex side of the coil is thus the direct 

 outcome of the impression produced by contact. 



Combined Effect of the Preceding Causes. The 



effects of light, heat, gravitation, etc., on growing plants 

 are thus seen to be manifold, and when considered sepa- 

 rately seem often conflicting and contrary to common 

 experience. The reason is that under natural conditions 

 the one influence counteracts the other, the growth of 

 the plant being the outcome of the combined effect of all 

 the causes alluded to, and of the operation at one time, 

 and under one set of circumstances, of the influence of 

 one agency (controlled or not by others), at another time 

 of a different agency. This affords an explanation of 

 the fact that the seasons marked by extraordinary pro- 

 ductiveness are not those wherein some one or more of 

 the conditions have been specially favorable at a particu- 

 lar time, even though that time be the growing period, 

 but those in which the conditions have been generally pro- 

 pitious throughout. The physiologist endeavors to isolate 

 the agencies which influence growth in order to ascertain 

 precisely what each does independently of the others ; 

 the practical man has to deal with the combined effect of 

 all, but it is clear that the combination cannot be 

 properly understood unless the separate effect of each 

 component be first clearly comprehended. 



