

BOTANY OF fHE LIVING PLANT 



a 



retain the power oi growth. When the shoot is placed in a horizontal 

 or oblique position the nodes resume growth on the under sides, 



with tin- result tli.it the- upper part of the shoot 

 again becomes erect (Fig. 96)- In this way 

 real plants which have been " layed " by 

 wind and rain stand up again if they are not too 

 old. Similar responses are to be seen in the 



Carnation. 



So far as leaves are concerned, the curvature 



,„ of the upper part of the stem brings the 



&rtrt younger leaves back into their normal position 



*2HS£S3& £2 with regard to gravity. Leaves on older parts 



| «<£*£ «t- have an independent capacity for adjusting 



their position by means of curvatures and 



torsions of the leaf stalk. In pulvinoid leaves, 



which will be considered later, these occur especially at the pulvini. 



2. Phototropism. 

 By Phototropism is meant the capacity of plant-organs to orientate 

 themselves in definite relation to the direction from which light 

 falls on them. Stems tend to orientate themselves so that they 

 point towards the source of light, and are said to be positively photo- 

 tropic. With overhead illumination the stem grows vertically : 

 lateral illumination frequently results in curvature (Fig. 97). The 

 curvature that develops in such a 

 laterally illuminated stem is again 

 due to differential growth, the cells 

 in the zone of elongation that are 

 away from the light tending to 

 elongate more rapidly than those 

 on the more brightly illuminated 

 side. The result is that the apical 

 part of the -tern is moved over until 

 it 1- dire* ted towards the source of 

 light. Before such curvature can 



■-. .^^:^^- •--.. v •••■•-..■. 



Fig. 97. 

 Phototropic curvatures in the hypocotyls of 



develop, the tendency of the shoot J|£ s ££ { f^ s) seedlin e s "^nated from 

 to remain upright as a result of its 



geotropism has to be overcome : the position which is finally 

 umed is the resultant of the operation of the two tropisms. The 

 same applies to other phototropic responses. 



