232 COLLEGE BOTANY 



same laws and the direction of s.tem growth, character and ar- 

 rangement of foliage, are influenced by the direction and in- 

 tensity of the sunlight. Trees growing near objects which 

 shut ofF the light from one side will not spread uniformly. Trees 

 growing in the open usually have short trunks and broad, 

 rounded tops, but trees grown in the forest mass must co'mpete 

 with each other for the light and therefore grow tall and 

 slender. The lower branches of the trees, growing in the dense 

 forest die prematurely and fall largely because they do not 

 receive the necessary sunlight. The grape and other climbing 

 plants are influenced in their direction of growth by the sun- 

 light ; when .growing in the forests they tend to grow tO' the top 

 of the tallest trees. Many plants that ordinarily grow and thrive 

 in the direct sunlight will make very tall, slender growths under 

 partial shade conditions. This principle is utilized in the grow- 

 ing of com and sorghum for fodder or silage, where it is desir- 

 able to develop small stems especially suitable for stock feed. 



Relationship of the Leaves to Sunlight. — The arrangement 

 of the leaves on the stem is correlated with the sunlight. The 

 alternate, opposite and whorled arrangements are such as to 

 bring the foliage into the most satisfactory position with regard 

 to the sunlight. The leaves on the tree are usually near the tips 

 of the shoots and thus form a canopy. The leaves on the tips 

 of these branches are usually so arranged as to produce a mini- 

 mum of shading. Looking down upon a tree, the leaves will be 

 seen to form a very excellent mosaic. The leaves of the Boston 

 ivy and similar plants are also arranged so as to produce a 

 mosaic, which is the best arrangement for the sunlight relation- 

 ship (Fig. 106). The form of the leaves and length of petioles 

 are also correlated with the sunlight needs of the plant. The 



