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 (Aabit 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 
