NATURE OF PLANTS 



29 



serve also that these lower leaves are larger and less lobed, thus 

 catching as much as possible of this rather feeble light (Fig. 18). 

 In many plants the lobing extends quite to the middle of the leaf 

 and the lobes are often attached to the midvein or midrib by a 

 petiole. In this latter case the leaf is said to be compound 

 (Fig. 19). All such modifications permit the development of 

 numerous leaves upon the branches without the danger of shad- 

 ing. If there is still any doubt as to the perfection of this light 

 catching arrangement of the leaves, try to substitute the some- 



FiG. 18. 



Fig. 19. 



Fig. 18. Leaves of red oak: A, sunned leaf. B, shaded leaf. 

 Fig. 19. Compound leaves: A, red ash. B, horse chestnut. 



what similar leaves of two different trees as the birch and elm 

 or the water beech (Carpinus) and the beech, noting how the 

 leaves of one fit when placed upon the branch of the other. 

 Another interesting feature about the leaf is its relation to the 

 intensities of light. Some plants demand the full intensities of 

 sunlight while others can tolerate only a small fraction of it. 

 Some lichens will grow in a light only 1/156 part of the full in- 

 tensity while many plants, as the grasses, will endure the strongest 

 illumination. Beech, maple and spruce are tolerant of shade and 

 oak, hickory and chestnut are intolerant. This relation of plants 



