DIV. n 



SPERMATOPHYTA 



585 



extremely small and weighs very little, seeds contain a certain 

 amount of reserve materials and are thus larger and heavier. In 

 spite of this the transport of seeds by the wind is the main means of 

 their dispersal. 



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

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

 Stanhopea, and the weight of a seed of Dendrobium attenuatum has been determined 

 to be about -5$^ milligramme. 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. 611), Poplar (Fig. 612), and 

 Cotton (Fig. 667,}, or a longer tuft of hairs is borne at one end as in the 

 Asclepiadaceae and Apocynaceae (Strophanthus, Fig. 745), and many Gesneriaceae 

 and Bromeliaceae. An equally frequent arrangement in other families of plants is 



Fro. 575. Winged seed of Pithecoctenium echinatum. (After NOLL. Nat. size.) 



the development of a flat wing formed of a thin and light membrane. This in our 

 Firs (Fig. 591) and Pines (Fig. 593) is split off from the ovuliferous scale, while in 

 Rhododendron, Bignoniaceae, some Cucurbitaceae (Zanonici), and in the Rubiaceae 

 (Cinchona, Fig. 766) it develops on each seed within the ovary. In no case is it 

 more perfect than in Pithecoctenium echinatum (Fig. 575), where 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, the large bract of the inflorescence of the 

 Lime (Fig. 669), the bract and bracteoles of Oarpinus (Fig. 605), and more 

 commonly the wall of the ovary as in Betula (Fig. 604), Alnus, Ulmus (Fig. 613), 

 Polygonaceae (Fig. 618 D), Acer (Fig. 684), Fraxinus (Fig. 739), or the fruits of 

 the Typhaceae, Eriophorum (Fig. 809) and Anemone (Fig. 641). 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 the Yalerianaceae (Fig. 769) and Compositae 

 (Figs. 780, 785), especially when it has a parachute-like form due to the later 

 elongation of the upper end of the fruit as in Taraxacum, Tragopogon, 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 Cynara Scolymus provided 



