42 



PEACTICAL BOTANY 



As a result of competition with each other to secure light, 

 plant stems often become greatly lengthened. Any one who 

 is observant and familiar with things out of doors must have 

 noticed the different form (habit it is called by botanists) of 



such plants as giant ragweed 

 (^Ambrosia) or hemp as they 

 grow tall and little-branched 

 when in dense clumps, or low 

 and spreading when they stand 

 singly. And full-grown trees 

 such as pines are nearly branch- 

 less for most of their height, 

 when growing in dense forests, 

 but low and broad-topped with 

 many lateral branches when 

 growing alone in a pasture 

 (Fig. 246). A tree growing on 

 the edge of a patch of dense 

 woods may develop the pasture 

 habit on its exposed side and 

 the forest habit on the side 

 toward the woods, like the tree 

 in Fig. 27. 



42. Danger from excessive 

 height of stems. Wheat, oats, 

 or corn plants are sometimes 

 blown down by severe winds, 

 and a field of grain in this con- 

 dition is said to be " lodged." 

 Large tracts of forest may also 

 be greatly damaged by severe 

 storms, particularly when the 

 trees are loaded with sleet (Fig. 28), and the area covered with 

 broken-down tree trunks is known as a " windfall." But neither 

 tall grain nor forest trees can be blown down as easily when 

 growing massed together as when standing singly, since every 



FIG. 27. Pruning as an effect of 

 shade 



The large American beech in the fore- 

 ground has developed no considerable 

 limbs on the right, because, until it 

 was well grown, another beech stood 

 within fourteen feet of it, on that side 



