830 THE DISTRIBUTION OF SPECIES BY OFFSHOOT& 



cause them to stick to the feathers of birds which come in contact with them as 

 they swim by. Others, such as the small Duckweeds (Lemna Tninor, gibba, 

 polyrrhiza), hang by their long, somewhat twisted, floating roots, and many 

 filamentous Algse, Aldrovandia, the delicate Riccias (Riccia nutans and Jluitans), 

 the Ivy-leaved Duckweed (Lemna trisulca), &c., become attached in their entirety 

 to the coot and duck swimming in the ponds and lakes. These fly away with them, 

 but as soon as they again enter other water the adhering plants fall off or are 

 cleaned off by the birds, and in this way they are distributed quite fi'esh and living 

 over great distances. We might also mention in this connection the peculiar distri- 

 bution of Ulvas, Floridese, and Sea-wracks by means of crabs, which was described 

 at vol. i. p. 77. 



We will only allude in passing to the fact that many economic plants are propa- 

 gated and distributed by offshoots to a veiy great extent by man. Bananas whose 

 fruits contain no fertile seeds. Potatoes, Artichokes, and many other tuberous and 

 bulbous plants are continually multiplied by the help of slips, tubers, bulbs, &:c. 

 The intentional artificial propagation by offshoots has of course no apparent 

 influence on the development of a natural method of distribution in such species. 

 Although planted and cultivated in large quantities they do not become naturalized ; 

 and if it were not for the artificial maintenance and propagation by offshoots they 

 would soon vanish again from such places, leaving no trace behind. This is, how- 

 ever, not the case with the unintentional distribution of offshoots of certain plants 

 by man. The keels and bottoms of ships journeying over wide seas become, like 

 the stakes and buoys of the harbour and the sea-walls and rocks of the shore, quite 

 overgrown with sea-weeds. If these are removed by chance or intentionally from 

 their substratum they do not necessarily perish. They may remain alive in the sea- 

 water, and under favourable conditions may attach themselves to some other firm 

 spot of ground. In this way plants may be transmitted from one coast to another 

 over very wide distances. Another unintentional distribution of plant-ofl'shoots by 

 man occurs on cultivated ground in vineyards, fields, and gardens. By ploughing, 

 digging, and throwing up the soil the bulbous or tuberous offshoots embedded in the 

 ground undergo a change of position. The offshoots of certain plants may in this 

 way be disti-ibuted so uniformly over a whole field by spade and ploughshare in the 

 course of a year that it almost looks as if they had been purposely planted there. 

 It is curious on journeying through the vine-planted districts of Northern Italy to 

 see one of two adjacent vineyards abundantly covered with wild Tulips, while not 

 one is visible in the other. In Central Europe the same thing happens with the 

 Gageas (Gagea arvensis, G. stenopetala) growing in the fields, and with the tuber- 

 forming Earth-nut pea {Lathy rus txiberosihs). One field looks as if it had been sown 

 with Gageas and yet its neighbour is completely devoid of them. On the Giinsel- 

 hohe in the Lower Austrian Erlafthal I once saw a rectangular ploughed field over- 

 grown from one end to the other with plants of the Bulbiferous Lily (Lilium biol- 

 bifh'UTn), while only isolated specimens of this plant could be seen in the adjoining 

 fields. There is no doubt that here the bulbils thrown on to the ground from the 



