126 



PHYSICAL FACTORS: LIGHT 



Some kinds of leaves are much more sensitive to light than other 

 kinds. Leaves that are not very sensitive to light, as those of the 

 tulip tree, Liriodendron hdipifera, are apt to assume a great variety 

 of positions with respect to light, but those that are more sensitive, 

 as those of the sugar maple, Acer saccharum, practically always have 

 their blades at right angles to the direction from which they receive 

 the greatest light, their petioles being twisted in such a way as to 



Fig. 60.— a larch tree the brandies of which have Ijeeii inu8tly self-pruned 

 from the side toward the forest but not from the side toward the street. A 

 light relation. 



bring this about. Some plants, as the common mallow, Malva 

 rotundifolia, have leaves that are so sensitive to light that their 

 blades turn toward the east in the morning and follow the sun to- 

 ward the west during the day, keeping the broad face always at 

 right angles to the source of light. 



77. Protection from Excessive Light.— Although all green plants 

 must have a certain amount of light, that which is very intense may 

 disintegrate chlorophyll. Most plants are able to protect themselves 



