SENSITIVENESS. 69 



reason of the impetus gained when circumstances were 

 more favorable. 



Influence of Heat and Moisture on the Stem, The 

 growth of the stem is directly influenced by heat, there 

 being in this as in other cases a minimum below which 

 growth cannot take place, an optimum at which it takes 

 place most vigorously, and a maximum beyond which 

 heat is injurious. The favorable influence of heat it is 

 which in part overcomes the influence of gravitation, and 

 enables the stem to ascend. The stem will grow fastest 

 and strongest on the side most exposed to the heat, if 

 that heat be not excessive, and this tendency will remove 

 it from the soil. Similarly a moist condition of the at- 

 mosphere favors growth, and the stem will grow the fas- 

 ter on the side most exposed to the moist vapor, and, 

 owing to the convexity so formed, it will in consequence 

 bend its free end and its concavity towards the drier side. 



Influence of Contact on Stems Climbing Plants, 



The most marked instance of this occurs in the case of 

 climbing plants. We have already seen that the young 

 growing parts of plants very generally exhibit gyratory 

 movements, these movements being produced by inequali- 

 ties 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 aifected by tempera- 

 ture 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 



