70 PLANT LIFE ON THE FARM 



growing parts of plants very generally exhibit gyratory 

 movements, these movements being produced by in- 

 equalities of growth, now in this direction, now in that, the 

 result being that the free end moves round, and that these 

 movements are only indirectly affected by temperature or 

 light. In the case of climbing plants, such as the hop, 

 the dodder, the tendrils of the pea or of the vine, which 

 are peculiarly sensitive to contact, these movements are 

 much more marked, the object being to secure a 

 suitable means of attachment, and so to expose the 

 leaves when present to the influence of light and air 

 with the least expenditure of force and tissue. Such plants, 

 in fact, depend upon others for their mechanical support. 

 When the free end of such a plant or a tendril 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, &c., on growing plants 

 are thus seen to be manifold, and when considered 

 separately 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 



