292 BOTANY 



wings, the centre of gravity is so disposed that the seed floats lightly 

 along through the air in an almost horizontal course, and with a 

 motion like that of a butterfly. The seeds of Zanonia, one of the 

 Cucurbitaceae, are very similarly equipped. In the Lime the subtending 

 leaf which is attached to the inflorescence is retained to facilitate the 

 dispersal of the seeds by the wind ; and in the seeds of the Fir the 

 winged appendages are derived from the tissue of the placental scale. 

 The aerial transportation of seeds and fruits, winged only on one side, 

 is accompanied by a continuous spirally twisting movement which 

 assists to retard their fall. 



The diminutive size of many reproductive bodies, and the propor- 

 tionate enlargement of their surface in comparison with their volume, 

 increase their buoyancy. Microscopically small Fungi, spores, and 

 Bacteria are in consequence easily transported by the wind. In the 

 spores of Lycoperdon caelatum Dingler found the retardation to be as 

 1 to 1000, which, according to Nageli, could only be theoretically 

 explained by the supposition that the retardation was intensified by a 

 thin layer of air permanently adhering to the surface of the spores. 



Seeds and fruit are also frequently transported great distances by 

 the agency of water. In the case of maritime plants the seeds are 

 often especially adapted (water-tight tissues; large air-spaces serving 

 as swimming-bladders, etc.) for transport by ocean currents. Through 

 the possession of such devices, the seeds of West Indian plants are 

 carried to Norway by the Gulf Stream, and the appearance of Cocoa- 

 nut palms as the first vegetation on isolated coral islands is in like 

 manner due to the adaptation of their fruits to transport by water. 



Animals participate largely in the dissemination of seeds ; either 

 by eating the agreeably tasting and often attractively coloured fruit, and 

 excreting the undigested seeds, or by their involuntary transportation 

 of seeds and fruits which have become in some way attached to them. 

 This is effected in many cases by hooks and bristles (Lappa, Galium 

 Aparine, Bidens, Echinospermwm, Xanthium, and the fruits of Medicago 

 minima, so common in sheep's wool and erroneously termed wool- 

 lice). Or the seeds become attached to animals by means of some 

 sticky substance ; in this way the seeds of the Mistletoe, which stick 

 to the beaks of birds eating the berries, finally adhere to the branches 

 of trees upon which the birds wipe their bills. The widespread 

 distribution of fresh-water plants can only be accounted for through 

 the agency of aquatic birds. 



The natural distribution of plants has been greatly modified by 

 the interference of man, especially in these days of universal commercial 

 intercourse by rail and sea. By their instrumentality not only have 

 the useful plants been widely distributed over the earth, but the weeds 

 have followed in the same way ; and many a seed thus accidentally 

 carried to other lands has finally found there a new place of growth. 



The forcible discharge of spores and seeds is effected by the 



