J07 



or in any other position, the hydrotropism of the root in this case Heiiotropism. 

 overpowering its geotropism. The aerial roots of some climbing 

 stems are also found to be sensitive to the direction of the rays 

 of light which fall upon them and they respond by growing 

 away from the source of light, i.e. they are negatively 

 heliotropic. Such roots being also positively hydro tropic and 

 very slightly, if at all, positively geotropic, turn towards the 

 moist dark crevices of their support and not downwards 

 towards the centre of the earth. 



The primary stem, in addition to being negatively geotropic, 

 is also sensitive to the direction of the rays of light falling on it 

 and responds by turning its apex towards the source of light, i.e. 

 it is positively heliotropic. In this case heliotropism is usually 

 more powerful than geotropism and if the stem is exposed to il- 

 lumination on one side it will grow obliquely towards the light 

 rather than vertically upwards. Most leaves are also sensitive 

 both to gravity and also to light, they being both dia-geotropic 

 and dia-heliotropic, but their position in nature depends mainly 

 on the direction of the light rays. As a rule they place them- 

 selves at right angles to the incident rays of light with their upper 

 surface turned directly towards the source of light. Plate XIII 

 for instance shows how the leaves of Coriaria nepalensis secure 

 this position on erect and horizontal branches, respectively. In sleep- 

 addition to this sensitiveness to the direction of the light rays, the mc 

 floral envelopes and also the foliage leaves of many plants are 

 found to be sensitive to variations in the intensity of light and 

 also to variations of temperature. Thus, with a rising tempera- 

 ture and light becoming more intense, many flowers open, while 

 they close with a fall of temperature and a diminished intensity 

 of light. The so-called sleep-movements of some foliage leaves 

 are due to the same cause, the leaves in this case assuming a 

 different position at niotfit to that taken up in the day, and as the 

 temperature falls, arid the intensity of the light decreases, in the 

 evening, these leaves, or leaflets, are found to close together and 

 to usuallyexpose only their edges to the zenith. In some cases it 

 appears that these movements are of service to the plant in pre- 

 venting excessive loss of heat by radiation. The so-called night- 

 position may also be taken up in some cases in the daytime, 

 owing to the light being too intense, or the temperature too great. 

 Many tendrils are found to be sensitive to contact with 

 rough solid bodies, as a result of which the side, in contact with 

 the substance grows slower than the opposite side, and the 

 tendril accordingly coils around the obstructing object, (unless 

 the latter is too large) and supports the plant stem. 



