CHAPTER III 

 LIGHT 



I. General Considerations. Importance of light in phyto-geography. 2. Photo- 

 metric Methods. Wiesner's work. 3. Plant-life in Darkness. 4. Intensity and 

 Quality of Light. Effects of light of various intensities on the different functions. 

 Harmfulness of light of high intensity and the corresponding means of protection. 

 Unequal action of difterent kinds of rays. Absolute and oecological optima of light. 

 5. Sun and Shade. Full light, light from above, light from the front, light from behind, 

 light from below. Direct sunlight and diffuse light. Wiesner's determination of the 

 actual photic ration of plants. Unequal demands made on light by sun-plants and 

 shade-plants. Contrivances for concentrating light possessed by shade-plants. 6. Day 

 and Night. Phyto-geographical significance of the unequal duration of daylight. Bonnier's 

 investigations with continuous illumination. 



I. GENERAL CONSIDERATIONS. 



Next to humidity light i.s the most important external factor affecting 

 plant-form. Whilst heat, which sets the plant-machine in motion and 

 during the whole course of its development and activity has a regulating 

 action of the first importance, yet has no essential influence in determining 

 its conformation, light on the other hand, like water, plays a prominent 

 part in controlling the structure of plants. A plant reared in the absence 

 of light is in form quite different from one that has been normally illuminated 

 and the structure is different at each degree of intensity of light. 



The phyto-geographical importance of light, however, in spite of its 

 powerful influence on the conformation and life of the plant, is less than 

 that of heat and rainfall because the supply of light in different climatic 

 regions varies less than that of these factors. Nevertheless until Wiesner 

 recently laid stress on the matter, its importance had usually been under- 

 estimated. The unequal intensity of the illumination in the different 

 climatic zones and the increasing duration of sunlight from the equator 

 to the poles do not fail to stamp their mark on the vegetation. The 

 importance of light however remains much greater in regard to botanical 

 topography, since the great diversity of illumination has much influence 

 in determining the characters of the several formations in a region. 



In the following pages only such effects of light as have real geographical 

 or topographical significance will be discussed. 



