SECT. II 



PHANEROGAMIA 



321 



Often the suitability of seeds for wind-dispersal is due simply to their niinute 

 size and their lightness ; thus millions of seeds are produced in a capsule of 

 StanhoiKM, and the weiglit of a seed of Dendrohiwm antennatum has been determined 

 by Beccari to be about -^l^ milligramme. The strongly hygroscopic pericarp 

 and the elater-like hairs assist in dividing up the mass of seeds so that the latter 

 are isolated, and this favours dispersal by the wind. Thus these orchids play a part 

 as epiphytes in damp tropical forests only equalled by Ferns, the spores of which 

 are as light. A much more common arrangement is found in heavier seeds when 

 the volume is increased and a large surface is offered to the wind. Either the 

 whole surface of the seed bears longer or shorter hairs as in the Willow (Fig. 529), 

 Poplar (Fig. 530), and Cotton (Fig. 597), or a longer tuft of hairs is borne at one 

 end as in the Asclepiadaceae and Apocynaceae {Strophanthus, Fig. 683), many 

 Gesneriaceae and Bromeliaceae and the fruits of Typhaceae, Eriophoru7n [¥ig. 745) 

 and A7iemone (Fig. 560). The same use is served by the crown of hairs (pappus) 

 which is developed at the upper end of one-seeded fruits such as those of 

 Valerianaceae (Fig. 717) and Compositae (Figs. 731, 737), especially when it has 



Fio. 492.— Winged seed oi PWhecoc.teniwm echinatmn. (Nat. size.) 



a parachute-like form due to the later elongation of the upper end of the fruit as 

 in Taraxacum (Fig. 732), Tragopogon, etc. An equally frequent arrangement in 

 other families of plants is the development of a flat wing formed of a thin and 

 light membrane. This in our Firs (Fig. 509) and Pines (Fig. 510) is split off from 

 the ovuliferous scale, while in Rhododendron, Bignoniaceae, some Cucurbitaceae 

 {Zanonia), and in the Rubiaceae {Cinchona, Fig. 711) it develops on each seed 

 within the ovary. In no case is it more perfect than in Pithecoctenium echinatum 

 (Fig. 492) when the delicate silky wing leads to the falling seed assuming an 

 almost horizontal position and being carried far even by a slight breeze. 



Other parts of the flower or fruit may be developed as wings, especially when 

 one-seeded fruits (or schizocarps) are concerned. Examples of this are afforded by 

 the sepals of the Dipterocarpaceae (either two, which are then much larger than 

 the others, or all five as in Dnjobalanops), the large bract of the inflorescence of 

 the Lime (Fig. 599), the bract and bracteoles of Carpinus (Fig. 521) and more 

 commonly the wall of the ovary as in Behola (Fig. 519), Alnus (Fig. 517), Vlmus 

 (Fig. 531), Polygonaceae (Fig. 547), Acer (Fig. 616), Fraxinus (Fig. 675), etc. 

 According to Dingler the fall in air as compared with that in a vacuum in the 

 first second is six times slower in the case of the fruits of C'ynara Scolymus provided 

 with scaly hairs ; in Pinus sylves^ris the fall is seven times and in Pithecoctenium 

 tJiirty times slower. 



