DISSEMINATION OF SEEDS. 127 



ripen, they become samarae, separate from the central 

 axis of the cone, and, being furnished with a wing, may 

 be easily wafted to a distance by the wind. 



Appendages or outgrowths of the testa, the outer cov- 

 ering of the seed, frequently have reference to dissemi- 

 nation by the wind. Such appendages belong to the 

 seeds of dehiscent fruits, also to the peculiar fruit of 

 cone-bearing trees. The wing of a pine seed, which is a 

 part of the carpellaiy scale upon which the two ovules 

 grow, is an example of such an outgrowth. The flowering 

 female catkin consists of bracts arranged in a spiral upon 

 an axis ; in the axil of each bract is formed a scale which 

 bears two ovules and which is therefore called a carpellary 

 scale. After fertilization the imbricated scales protect 

 the ovules as carefully as would a closed pericarp. The 

 cones remain closed on the trees for at least one season 

 after fertilization. When the hot weather of the next 

 summer has continued for some time so as to dry the 

 cones thoroughly, the scales open with a sudden spring, 

 ejecting the seeds which, being rendered buoyant by the 

 whig before-mentioned, are committed to the wind that 

 the seeds may be borne away and the range of the species 

 extended. If a number of these cones happen to open 

 together, the noise of the cracking can be heard to a 

 considerable distance. 



Alders and birches which are so abundant in the cool 

 regions of the northern hemisphere produce cones similar 

 to those produced by pines, spruces, and larches, yet, 

 in some respects, unlike them. The seeds of the birch 

 become winged and scale-like nutlets, or small samara?. 

 These seeds are often carried by the wind to the lofty 

 summits of rocks and mountain peaks and, for this reason, 

 birches are found in all parts of northern Europe, Asia, 

 and America where heavy seeds like those of the oak and 

 the hickory cannot follow. 



