DISPERSAL BY WIND. 



851 



This brings us back to the fruits of Mesembryanthemum and Anastatica, which were 

 described on pp. 845, 846. Sometimes these also play the part of rolling fruits. 

 The capsules of Mesembrianthemum detach themselves from their stalks, the plants 

 of Anastatica become partially uprooted, and lie during the dry season of the year 

 loose upon the earth. A puff of wind blows them into hollows in the ground or 

 cracks in rocks, where they are held prisoners. The seed-cases, however, still 

 remain closed. At last the winter rains set in, whereupon the capsules open, the 

 seeds are washed out, and after a short time they germinate on the saturated 

 ground, to which the rain has conveyed them. 



Innumerable are the cases of wind-dispersed fruits and seeds which remain 

 floating in the air for a period of more or less duration after severance from the 

 mother-plant, and which have their fall retarded by special contrivances for the 

 purpose. The conformation of fruits and seeds of this category must be such that 

 the air offers great resistance to their fall, and it is important that they should 

 possess as small a weight as possible in relation to their size. It is well known that 

 the spores of Fungi often remain for a long time floating in the air as constituent 

 particles of the dust. Some seeds, too, are so extrao^■dinarily light that they also 

 look simply like dust and are able to remain for a comparatively long period sus- 

 pended in the air. Amongst such dust-like seeds those of Orchids must be men- 

 tioned first. A single seed of Ooodyera repens, for instance, weighs only 0-000002 

 grm. Several other plants, particularly parasites and saprophytes which live in 

 deep beds of humus, possess extremely light seeds, as is shown by the annexed 

 table: — 



To enable these seeds to float in the air for as long a time as possible they are 

 more or less flattened, and their centre of gravity is so placed that they always 

 present the broad side to the direction of descent. The same form of adaptation 

 occurs in seeds which are shaped like leaflets, scales, or delicate discs. A compressed 

 seed is usually surrounded by an attenuated margin, a membranous border, or a 

 radiating fringe of extremely fine processes, as in FunJcia, Lilium, Tulipa, Fritil- 

 laria, Rhinanthus, Veronica, Lepigonum, Cinchona, Bignonia, Dioscorea, and 

 Heliosperma (see p. 423, figs. 318 *■ ^' ^ and figs. 466 ^- *• ^). In some cases the entire 

 pericarp is modified in this manner, as in Hymenocarpus, Mattia, Peltaria, Ptelea, 

 and Ulmus (see fig. 467 *, and p. 143, fig. 232 ^). Amongst Umbelliferae, Mimoseae, 

 Papilionacese, and Cruciferse cases also occur in which the mericarps, the segments 

 of siliculas and lomenta, or the seed-studded valves of ordinary pods and siliquas, 



