60 INTRODUCTION TO BOTANY 



where they can cover the surface of the ground closely enough 

 to prevent too severe competition from plants that might shade 

 them. 



As a result of this competition for light, plant stems often 

 become greatly lengthened. Any one who is observant and 

 familiar with things out of doors must have noticed the dif- 

 ference in form (habit it is called by botanists) of such plants 

 as giant ragweed {Ambrosia trifida) and hemp ; they grow 

 tall and little branched when in dense clumps, but low and 

 spreading when they stand alone. Full-grown trees, such as 

 pines, are nearly branchless for most of their height when 

 growing in dense forests, but are low and broad-topped, with 

 many lateral branches, when growing alone in a pasture. 



58. Plants blown down by wind. Most farmers who grow 

 any kind of grain have had losses from lodged grain that 

 is, from crops which have been more or less completely blown 

 down by a windstorm, and especially by a wind accompanied 

 by a heavy rain. Forest trees are often blown down by severe 

 winds (fig. 41). Where they pass through forest tracts, the 

 violent rotary storms commonly known as cyclones frequently 

 leave behind them windfalls, in which the tree trunks lie in 

 piles for long distances. Individual plants of any kind of 

 grain, and tall, slender forest trees growing under usual con- 

 ditions, are greatly protected by their neighbors. The whole 

 mass of plants, standing as close together as they do, inter- 

 cepts much of the wind, so that the single plant is exposed 

 to only a small fraction of its total force. 



59. Growth in length of stem. Under favorable conditions 

 the younger regions of the stem continue for some time to 

 increase in length. The rate of growth varies greatly in 

 different plants : the giant ragweed and certain kinds of sun- 

 flowers may grow to a height of 10 or 12 feet, and climbers 

 like gourds and hops, to a length of perhaps 40 feet, in a 

 single summer. On the other hand, pine seedlings, during 

 their first summer, grow to be only from 1 to 3 inches high, 

 and oak seedlings less than 5 inches. For a time the growth 



