6 



climbing roots are not sensitive to gravity as they have 

 a very definite function to perform in fixing the plant to 

 the wall, and are not concerned in the absorption of food 

 material. 



Wherever bending, such as described above, takes place 

 in growing organs, this is due to differences in the amount 

 of growth on different sides of the stem or root. The con- 

 cave side grows less than the convex side. Thus when a 

 stem, which has been laid horizontally, bends upwards, 

 this is due to the greater amount of growth of the side 

 nearer the ground. If a stem illuminated from one side 

 bends permanently in that direction this is due to the fact 

 that light retards the rate of growth and the side away 

 from the light growing more rapidly the stem becomes 

 convex on this side and bends tow^ards the light. The 

 fact that light retards growth and therefore causes plants 

 to be short and " stocky " is of course a well-knowTi 

 phenomenon, while the lack of illumination acting like 

 darkness causes more rapid growth and we get long 

 " leggy " plants, when they are insufficiently lighted as 

 when grown in the shade, in deep frames or pits, or not 

 close up to the lights in greenhouses. 



There are one or two other factors influencing the 

 growth of plants which it may be useful to refer to at this 

 juncture. Besides being sensitive to gravity roots are 

 also sensitive to contact, and when a root tip comes in 

 touch with a solid body such as a stone in the soil it bends 

 away from it. This is brought about as in the case of 

 other bending movements, by the fact that contact causes 

 retardation of growth on this side of the root and this side 

 becomes therefore convex, the root-tip pointing away from 

 the obstacle met with in the soil. In this way it is possible 

 for the root to make its way even through a stony soil, 

 avoiding or rather growing round all obstacles with which 

 it may be met, bending to right or to left in its progress 

 downwards. This sensitiveness to contact is however not 

 only possessed by main roots which grow dowTiwards ; it 

 is equally important to lateral roots. 



It has been found in the somewhat infrequent cases 

 occurring in nature that where the soil is drier on one side 

 of a plant than on the other the root system develops 

 more abundantly in the moister soil. Experimentally, too, 

 we can prove that a root will grow towards moisture just 

 as the stem of a plant will grow towards the light. 



