636 The Outline of Science 



as gusts of wind lift and scatter. Still more effective are the 

 tufts of hair which crown the fruits of dandelion and thistle with 

 down, or the seeds of willow, cotton, and willow-herb. A light 

 fruit like that of the groundsel is sustained in the air by the 

 gentlest of currents. It has even been suggested that the eddy 

 caused by the sun's rays striking the white tuft, and warming the 

 atmosphere round it, is sufficient to lift the seed in perfectly 

 calm air. 



It is not surprising that plants which drift in currents of 

 water and air should be widely distributed. The duckweed, float- 

 ing in slow streams, carried on the feathers of aquatic birds, is 

 cosmopolitan. The groundsels, an immense genus with over 2,000 

 species, took their origin in the Bolivian Andes. Evolving new 

 species on their way, they travelled along the great mountain 

 chains till they covered the earth. Beside such wanderings the 

 feats of a Marco Polo, even of a De Rougemont, pale. 



Vegetative Multiplication 



Gardeners often propagate plants by methods which dis- 

 pense with seeds. They use slips and cuttings, they graft and 

 bud. The success of the cutting depends on the ready formation 

 of roots from near the cut surface; to go a little deeper, it 

 depends on the loose integration of the plant's organs, on the 

 power of independent existence possessed by separated parts. 

 In the case of grafting there must be added the faculty of uniting 

 with the tissues of the stock. This is not true reproduction ; here 

 is no fresh start from a fertilised egg-cell. We may call it vege- 

 tative multiplication. 



In nature similar processes are of frequent occurrence. The 

 runners of bugle or silverweed take root and grow into new 

 plants, separated in time from the parent by the death of the 

 connecting pieces. Rhizomes of the bracken, anemone, or couch- 

 grass, branch, and the branches become separate plants. Bulbs 

 and tubers, as of the scilla and lesser celandine, bud off young 



