SENSITIVENESS. G9 



retarding influence of growth, which is so manifest when 

 the plant is grown under artificial conditions, when 

 the influence' of other agencies is prevented or ex- 

 cluded, is compensated for or overcome hy other agencies 

 temperature, moisture, &c. when the plant is grown 

 under natural conditions. Again, what is called the 

 " after effect " has to be considered the facilities for 

 growth afforded by the absence of light, by the agency of 

 heat, or other forces, may continue after those influences 

 have ceased to act, and so a plant may grow for a time under 

 adverse influences by reason of the impetus gained when 

 circumstances were more favourable. 



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 favourable 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 

 atmosphere favours growth, and the stem will grow the 

 faster on the side most exposed to the moist vapour, 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 Flaats. 

 The most marked instance of this occurs in the case of 

 climbing plants. We have already seen that the young 



