THE DISPERSION OF SEEDS AND SPORES. 77 



The three wings of the beech (Fagus) are slightly 

 twisted — the fruit itself might indeed be compared 

 to a rifle-bullet with its spiral grooves and ridges ; 

 and a well-developed samara of the sycamore (Acer 

 Pseudoplatanus) resembles uncommonly the screw- 

 propeller of a model steamship. 



The hairs or plumes with which so many seeds are 

 furnished constitute an appliance better adapted 

 for dispei'sion than even the wing-like expansions just 

 considered. Hairs, if horizontal, prevent a seed 

 from falling quickly, and keep it afloat in the air ; 

 if vertical, they act like sails, exposing a large 

 surface, so that the seed is impelled rapidly before 

 the wind. Sometimes the entire outer surface of 

 the seed is beset with long silky hairs, as in the 

 cotton-plant (Gossypium) belonging to the Malvaceae. 

 In the allied Sterculiaceae, or silk-cotton order, 

 several genera, including Bomhax, Eriodendron, etc., 

 possess the same peculiarity. More frequently, how- 

 ever, the hairs are confined to one end of the seed, 

 and form a tuft, as in Salix and Populus, where 

 they grow from the funiculus and constitute an 

 arillus. AscLepia, Apocynum, Taniarix, and Epilo- 

 hhim have an arillode, the hairs springing from the 

 micropyle of the seed. Hibiscus and Alstonia have 

 seeds tufted at both ends. In Anemone Pulsatilla, 

 and several species of Clematis, Dryas, and Geum, 

 as well as in the plume-nutmeg (Atherosperma), 

 hairs are developed from the pericarp. The pappus 

 of Compositse and Valerianacese may be considered 

 calycine. Hairs are developed from the flower-stalk 

 in Eriox>horum, and Typha, while several grasses 

 have them on the bracts. 



The pappus of Compositae exhibits considerable 

 variety. In the thistle (Carduus lanceolatus), and in 

 Hypochceris, it is feathery or plumose ; in Crepis 

 and Sonchus, pilose. The pappus of Tragopogon 

 and Taraxacum is provided with a beak or stalk ; 

 that of Hieraciuni is sessile. These and other minor 

 differences are not without significance. The special 



