78 THE MECHANICAL FUNCTIONS. 



ance of stomata. They increase in thickness in the same 

 way as the stem increases. This law obtains both in exo- 

 genous and endogenous plants: they do not, however, grow 

 in length by the elongation of any of their parts, but simply 

 by additions made to their extremities. Their ramifications 

 are not the result of the development of buds, as are the 

 branches of the stem; but they arise merely from the addi- 

 tional deposites taking different directions. Almost every 

 part of the surface of the stem or branches may shoot forth 

 roots if they are covered with earth, and properly moistened, 

 and if they are supplied with sap from the circulating system 

 of the plant itself. It is observed, however, that they gene- 

 rally grow from certain points on the surface of the bark, 

 which appear as dark spots, and are termed Lenticellss.^' 

 Great variety exists in the form, and disposition of roots in 

 different families of plants, according to the particular pur- 

 poses they are intended to serve, conform.ably to their ge- 

 neral functions of absorption and of mechanical support. 

 Both these purposes are promoted by their sending out from 

 their sides numerous fibrils, or lesser roots, which increase 

 their firm hold upon the soil, as well as multiply the chan- 

 nels for the introduction of nourishment. 



Nature has supplied various plants with certain appen- 

 dages to the above mentioned structures, the use of which 

 are for the most part sufficiently obvious. Of this descrip- 

 tion are the tendrils, which assist in fixing and procuring 

 support to the stems of the weaker plants; the stipidde, 

 which protect the nascent leaves; and the hractese, which 

 perform a similar office to the blossom. The different kinds 

 of hairs, of down,t 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 defend- 

 ing the plant from molestation by animals. The sting of 

 the nettle is of this class; and its structure bears a striking 



* This name was g^iven to them by De Candolle, Annates des Sciences 

 Naturelles, VII. 1, and Organograpliie, I. 94. 



f The finer hairs, and filaments of down, are composed of elongated cells, 

 either single, or several conjplned end to end. 



