SENSITIVE-PLANTS. 59 



the domain of vegetation to the farthest possible limits. 

 The branches, too, send down slender roots like those of 

 the banyan, and become independent trees. Thus a 

 complete woody labyrinth is formed ; and the network 

 of tough roots and stems resists the action of the tides, 

 and enables the mud brought down by great tropical 

 rivers to be converted into solid land far more rapidly 

 than it could be without this aid. 



Sensitive- plants. Among the more humble forms of 

 vegetation that attract the traveller's notice none are 

 more interesting than the sensitive species of Mimosa. 

 These are all natives of South America, but one species, 

 Mimosa pudica, has spread to Africa and Asia, so that 

 sensitive-plants now abound as wayside weeds in many 

 parts both of the eastern and western tropics, some- 

 times completely carpeting the ground with their 

 delicate foliage. Where a large surface of ground is 

 thus covered the effect of walking over it is most 

 peculiar. At each step the plants for some distance 

 round suddenly droop, as if struck with paralysis, 

 and a broad track of prostrate herbage, several feet 

 wide, is distinctly marked out by the different colour 

 of the closed leaflets. The explanation of this pheno- 

 menon, given by botanists, is not very satisfactory ; ' 

 while the purpose or use of the peculiarity is still 

 more mysterious, seeing that out of about two hundred 

 species belonging to this same genus Mimosa, only 

 some three or four are sensitive, and in the whole 

 vegetable kingdom there are no other plants which 

 possess more than the rudiments of a similar property. 



1 See Nature, vol. xvi. p. 349, where the German botanist Pf offer's theory 



is given. 



