Factors Affecting Growth and Reproduction 215 



W. S. Cooper 

 Fig. 125. Limber pine {Pinus flexilis) at timber line on Long's Peak, Colorado, showing 

 effects of environmental factors on growth. When exposed to the wind, snow, and low 

 temperature of the mountain peak, the tree has the scraggy, much-branched form shown 

 in the illustration. At lower altitudes it is a single-stemmed timber tree, 



vailing winds increase transpiration and slow down growth on the 

 windward side of trees to such an extent that a larger part of the 

 crown of any tree standing in the open is on the leeward side of 

 the trunk. This is so generally true that one can tell the direction 

 of the prevailing winds of a region by a careful examination of the 

 trees. Occasionally violent winds may destroy large areas of 

 timber and crops, and along exposed coasts and mountain tops 

 bring about the development of stunted and gnarled trees. 



Gravity. The direction of growth of many plant organs is 

 determined by gravity. The downward growth of primary roots, 

 the upward growth of stems, and the direction of growth of 

 lateral branches are responses to gravity acting as a stimulus. 



The peculiar shapes of York Irtiperial apples are the results of 

 gravity stimulating growth in a vertical direction, no matter 

 what the position of the axis, or core, of the apple. If the apple 

 hangs vertically downward during growth, the mature fruit will 

 have a long, sheep-nosed form. If it extends horizontally from 



