SECT. II 



PHYSIOLOGY 



319 



Bignoniaceae and many Terustroemiaceae). In a seed of one of the Bignoniaceae 

 (Fig. 248), with its widely outspread, glossy 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 

 fruits 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. Thus in the above-mentioned 

 Bignonia DINGLER found the retardation in the first second amounted to thirty 

 times the free fall, and in Pinus silvestris to seven times. 



The diminutive size of many reproductive bodies, and the proportionate 

 enlargement of their surface in comparison with their volume, increase their 

 buoyancy. Microscopically small Fungi, spores, and Bacteria are in consequence 



FIG. 248. Winged seed of Pithtcocteniuni echinatnnt. (Nat. si/i*.) 



easily transported by the wind. According to FALCK'S observations even the heat 

 produced by the respiration of a fructification of a fungus is sufficient to suspend 

 the spores in the air. In the spores of Lycopcrdon caelatum DINGLER found the 

 retardation to be as 1 to 1000, which, according to NAGELI, could only be theoretic- 

 ally explained by the supposition that the retardation was intensified by a thin 

 layer of air permanently adhering to the surface of the spores ( 121 ). 



Minute dust-like spores and seeds form the most effective means of dispersal, as 

 was shown in the return of vegetation to the island of Krakatoa, where the flora 

 was destroyed by a volcanic eruption in 1883. Following the forms with minute 

 seeds or spores came those with hairy fruits or seeds, and later than these the 

 winged seeds. 



Seeds and fruit are also frequently transported great distances by the agency of 

 VVATI.K. In the case of maritime plants the seeds are often especially adapted 

 (water-tight floating tissues (Fig. 249) ; 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, as was known to LINNAEUS, and the appearance of Coco-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. SOHIMFEB was able to show in detail the value 

 of ocean-currents for the geographical distribution of the strand-flora ( 122 ). 



ANIMALS participate largely in the dissemination of seeds ; either by eating the 



