848 



THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



those circumstances that can supply tlie propelling force necessary to drive such 

 fruits and seeds as can keep afloat from one shore to another. Special mention 

 must be made of three groups of fruits and seeds belonging to this categoiy. These 

 are, firstly, dry fruits which are rendered buoyant by air-inclosing envelopes, as, for 

 instance, in the case of the marsh-plants known as Sedges (Carex ampullacea, 

 C. vesicaria, &c.), where the fruit is surrounded by an inflated utricle; secondly, the 

 fruits of Water-Plantains, Flowering-rushes, &c. (Alisma, Butomus, Sagittaria, 

 Sparganiuni, &c.), which are furnished with a thick air-filled ccn-tical parencliyma; 

 and, thirdly, the seeds of some Water-lilies. In the case of the white Water-lilies 



464.— Dispersion of fruits and seeds by the wind. 



I Anthyllis Vulmraria; two fruiting calyces are falling from the plant. 2 Longitudinal section through a fruiting calyx 

 belonging to the same plant; the pod is visible in the interior. 3 Trifolium tomentositm ; one head of inflated fruiting 

 calyces is detached, and another is still attached to the stalk. * Longitudinal section through a fruiting calyx belonging 

 to the same. ^ Medicago sciUeltata. ^ Ostrya carpini/oUa; branch with two fruit-spikes. 7 Longitudinal section through 

 the saccate cupule which envelops the nut in this plant 



(Nymphcea), each seed is enveloped in a coat (arillus), which loosely clothes the 

 outer integument (testa) of the seed, so as to leave a layer of air between the two. 

 In the species of the genus Nuphar there is no arillus, but the carpels separate 

 when the fruit is ripe into two layers, of which the outer one is green and succulent, 

 whilst the inner one is white and chai-ged with air, and incloses a large number of 

 seeds. In all these cases the seeds are enabled to float by their envelopes, and are 

 driven along on the surface of the water by the wind. 



In a similar manner the wind causes certain detached as well as aggregated fruits 

 to roll along upon level ground. This phenomenon is observed particularly in regions 

 where a long period of drought follows the short summer season of development; 

 and accordingly the plants concerned are especially abundant in the vicinity of the 

 Mediterranean Sea and in Steppe-lands. Several Umbellifers indigenous to the high 



