258 The Living Plant 



curves that permit the smaller to stream with the wind in the 

 lee of the larger, where they can tug at their anchorage in safety. 

 Doubtless in a windless world the plant skeleton would be rigid 

 and brittle, probably to such a degree that an ordinary one of 

 our storms would shatter it to fragments, much as at times they 

 do now with the ice of a silver thaw. As to older stems, we have 

 learned already how it is with them; their hollow-column princi- 

 ple of construction holds them up against great lateral strains. 

 Furthermore, a good many kinds of stems exhibit a special 

 strengthening arrangement at the place of maximum weakness, 

 which lies at the contact of stem and root, where the leverage 

 exerted by wind on the top is most felt. Thus, some kinds of 

 plants, like the Corn, develop prop roots that extend from the stem 

 above ground diagonally down to the earth, while many tall 

 trees possess buttress-like thickenings between the stem and the 

 principal roots, as appears very well in some of our Elm trees, 

 and especially in some of the tropical giants, where they attain a 

 good many feet of height and breadth, though only a few inches 

 of thickness. As to leaves, whose broad faces would present 

 much exposure to wind, their slender-elastic petioles permit them 

 to yield, and to swing like so many weather vanes, presenting 

 only edges to the blast, w^hile they can also sway accommoda- 

 tingly to every irregular gust. In this adaptation, indeed, we 

 find one of the principal functions of the petiole, as follows from 

 a discovery made by one of my own students, — w^ho found that 

 the petioles from the exposed part of a tree average longer than 

 those from more sheltered situations, although the leaves are 

 smaller in the former locations than the latter. 



But it is not alone on the individual tree that the sizes of leaves 

 are inversely proportional to the degree of their exposure to 

 winds, for it is true in general of plants as a whole. Do not the 

 largest leaves that are known to the reader grow in the shelter 

 of undergrowth? And if at first sight it appears that the gigantic 

 fronds of Palms and Tree Ferns contradict this view, a second 



