2S6 GROWTH 



These movements go on in the dark, and are the result ol 

 stimulus of gravity acting upon the sensitive tips of the stems 

 and roots and not directly upon the growing parts which become 

 curved. 



The lateral secondary branches of roots and stems appear 

 to be less sensitive to the action of gravity than primary 

 members ; for example, secondary roots grow obliquely and 

 not vertically downwards in the soil. 



The peduncles of most flowers are generally apogeotropic but 

 in some cases their geotropic irritability changes when the flower 

 opens : many varieties of daffodil become diageotropic when the 

 flower opens, the ' trumpet ' of the corolla then taking up a more 

 or less horizontal position. 



The stems of wheat, barley and grasses generally curve up- 

 wards at the nodes when they are bent on one side by the wind 

 and rain, and the upper internodes and ears may eventually 

 attain an erect position after the crop has become ' laid,' if the 

 latter does not happen too late in the season. 



This apogeotropic movement of a cereal stem is due to the 

 stimulus of gravity which induces a renewal of growth in the 

 tissue forming the swollen leaf-bases close to the nodes. 



Ex. 159. — Repeat Ex. 9, and note the geotvopic behaviour of the roots 

 and stems of the beans employed. 



Ex. 160. — Sow a runner bean in a pot of garden soil and keep in a dark 

 place. When the stem of the seedling is two or three inches long turn the 

 pot on its side so that the young stem is horizontal and leave it to grow in 

 the dark as before. After a few hours examine and note the curvature of 

 the stem : which part has curved most ? 



Ex. 161. — Cut a straight piece of -- young barley or wheat stem with a 

 node about the middle of it, and place the lower cut end through a hole in a 

 cork which fits into a small flat medicine bottle. P'ill the bottle with water, 

 insert the cork with the straw through it, and place the bottle on its side so 

 that the straw is horizontal. Leave it in a dark cupboard all night and 

 examine next morning. Is the straw still horizontal ? 



(v) Movements induced by variations in moistness of the 



