390 The Living Plant 



weight of the seed which hangs some distance below. Thus the 

 seeds can be lifted over the tops of the trees by those currents of 

 air which rise upward from places that are heated in summer, 

 whence the wind may transport them to far distant parts. It is 

 largely, no doubt, because the Dandelion, and its relatives in the 

 family Compositae, have developed so efficient a mode of wind 

 transport, that they constitute the largest and most widely 

 diffused of all the existent plant families. And it is also of interest 

 to note that these wind-carried seeds exhibit a very effective 

 secondary adaptation to wind-transport, namely, great lightness 

 of build, which even extends to the employment of oil, a lighter 

 material than starch, as food in reserve for the embryo. And 

 other modifications of this principle of plume-transport exist, 

 as the reader may learn from his own observation, or the several 

 good books on the subject. 



If one compares the habits of plants whose seeds are winged or 

 are plumed, respectively, he will find that in general the winged 

 seeds belong to trees and the plumed seeds to herbs. It is easy to 

 imagine a reason for this. Plumes are a better lifting device than 

 are wings. The extra height and better exposure to wind of the 

 tree gives its winged seeds a start which is ample for transport to 

 sufficient, if not to the greatest distances. Moreover, some tree 

 seeds possess such a relation of weight to wing form that they do 

 not fall directly to earth, but only after whirling through long 

 spirals, thus giving the wind a longer action upon them. But 

 with herbs in their low sheltered positions, the wing would be far 

 less efficient; and the lifting action of plumes is a notable ad- 

 vantage. The plume is actually a better device than the wing, 

 and there is reason to think it a later evolutionary development ; 

 for our modern herbs, I believe, appeared later in time than our 

 trees. 



There is still another method, quite different in principle, of 

 increasing the surface in proportion to weight. It consists in 

 excessive reduction in size. It is a mathematical fact that as a 



