I INTRODUCTION 19 



depressions in the carpels. In other cases, perhaps, they 

 serve to promote dispersal by diminishing the specific 

 gravity of the seeds. It has been suggested that the 

 smooth surface of many seeds is perhaps an advantage 

 by enabling them to pass through the body of an animal 

 without being digested. Another explanation may be 

 that seeds which are thrown by the plant, or jerked by 

 the wind, would no doubt go further if the surface was 

 smooth, because it would offer less resistance to the air. 



It may be convenient to say a few words as to the 

 fruits of trees with reference to the dispersal of the 

 seeds. Nature, as we have already seen, has devised 

 several plans for the dispersal of fruits and seeds. For 

 instance, some are edible, and are carried by animals ; 

 some are hairy or winged, and are carried by currents 

 of air ; some have hooks, and become entangled in fur 

 and hair. 



That these differences of structure are really adapta- 

 tions comes out clearly if we take some definite group, such 

 as our common forest trees, shrubs, and tall climbing 

 plants ; not, of course, a natural or botanical group, 

 for they belong to a number of different orders, but 

 a group characterised by attaining to a height of, 

 say, over eight feet. We will in some cases only 

 count genera ; that is to say, we will count all the 

 Willows, for instance, as one. These trees and shrubs 

 are plants with which we are all familiar, and are about 

 thirty-six in number. Now of these thirty -six no 

 less than twenty-two have edible fruits or seeds, such 

 as the Plum, Apple, Arbutus, Holly, Hazel, Beech, 

 and Rose ; three have seeds which are provided with 

 feathery hairs ; and all the rest, namely, the Lime, 

 Maple, Ash, Sycamore, Elm, Hop, Birch, Hornbeam, 

 Pine, and Fir, are provided with a wing. Moreover, as 

 will be seen by the following table, the lower trees and 

 shrubs, such as Cornel, Guelder Rose, Rose, Thorn, 

 Privet, Elder, Yew, and Holly, have generally edible 

 fruits, much eaten by birds. The winged seeds or 

 fruits chara,cterise the great forest trees. 



