82 THE MECHANICAL FUNCTIONS. 



Nature has supplied various plants with certain 

 appendages to the above mentioned structures, the 

 uses of which are for the most part sufficiently 

 obvious. Of this description are the tendrils, which 

 assist in fixing and procuring support to the stems 

 of the weaker plants ; the stipules, which protect the 

 nascent leaves ; and the bracfece, which perform a 

 similar office to the blossom. The different kinds 

 of hairs, of down,* of thorns, and prickles, which 

 are found on the surface of different plants, have 

 various uses; some of which are easily understood, 

 particularly that of defending the plant from mo- 

 lestation by animals. The sting of the nettle is of 

 this class ; and its structure bears a striking analogy, 

 as we shall have afterwards have occasion to notice, 

 to that of the poisonous fangs of serpents. 



The purposes answered by the down, which 

 covers a great number of plants, are not very obvious. 

 It perhaps serves as a 23rotection from the injurious 

 effects of cold winds on the tender surface : or it 

 may have a relation to the deposition of moisture ; 

 or, as it may be farther conjectured, the number of 

 points which are thus presented to the air may be 

 designed to convey electricity from the atmosphere, 

 or to restore the electric equilibrium, which may 

 have been disturbed by the processes of vegetation. 



In the smaller parts of plants, as in the general 

 fabric of the whole, we find, on examination, the 

 most admirable provision made, according to the 

 particular circumstances of the case, for the me- 

 chanical objects of cohesion, support and defence. 

 Thus the substance of the leaf, of which the 



* The finer hairs, and filaments of down, are composed of elon- 

 gated cells, either single, or several conjoined end to end. 



