vii) PLANT. LIFE ON 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. Asa 
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 : as 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. 
