358 



A TEXTBOOK OF BOTANY 



[Ch. VII, 3 



Fig. 252. — Fruit 

 of American Elm, 

 containing one seed ; 

 enlarged. (From 



Bailey.) 



adaptational features frequently, perhaps mostly, arise by 

 the development of features originally incidental. 



3. Waftagc by winds. This is the commonest of the 

 methods of dissemination, and the most efficient for wide 

 disper.sal. Fruits and seeds often develop 

 a large spread of surface in proportion to 

 bulk, thus giving a hold to the wind which 

 wafts the seeds to a distance. Often this 

 feature is a "wing," a thin flat plate de- 

 veloped from the ovary, as in Maple or 

 Elm (Fig. 2.52), or from the seed-coat, as 

 in Bignonia (Fig. 2.53), or from part of a 

 cone scale, as in Pine (Fig. 249), or from a 

 bract, as in Linden (Fig. 47). That the 

 plants here mentioned are all trees is not 

 mere coincidence but typical of the fact 

 that winged fruits or seeds are almost 

 confined to trees or high-climbing shrubs, while the same 

 is true of kinds having ver3' flat pods, as in Locusts. 

 On the other hand, the fruits and seeds of herbs and low- 

 growing shrubs more often have hairs or plumes, either in 

 great profusion, as in Cotton (Fig. 254), or in terminal tufts, 

 as in Milkweed (Fig. 2.55), or in parachute-like arrange- 

 ments, as in Dandelion (Fig. 256); r--,;^ 

 or in yet other waj^s. Such structures 

 give a hold to the wind, which per- 

 mits not simply a lateral transport 

 of the seeds before they reach the 

 ground, as occurs and suffices in the 

 case of trees, but a lifting action 

 whereby even light breezes raise the 

 seeds to a height whence winds may 

 carry them far. It is notable that plants of this kind, as 

 witness the Dandelion, are among the most widespread and 

 abundant of plants. The plumes are of diverse morpho- 

 logical origins, being special cnitgrowths from seed coat 



F'' \ 



Fig. 253.^ Seed of Big- 

 nonia alba-Iiitta. Reduced. 

 (From F. ITarwin.) 



