THE ROOTS OF VASCULAR PLANTS. 223 



remarkable example of this phenomenon in the Eryngo, 

 and I have especially verified it in Eryngium maritimum, 

 which grows in the sand on the sea-shore ; its stem is 

 sometimes buried several feet in depth, and takes 

 throughout the whole of this length the appearance of 

 a root ; the same thing happens in the Echinophora in 

 the same situation, and I have mentioned above the ex- 

 ample of the Salix herhacea, which, by the elevation of 

 certain Alpine soils, becomes a kind of subterranean 

 tree ; in these different cases the direction of the 

 branches towards the upper side is the character by 

 which we can most readily distinguish these buried 

 stems from true roots. 



Section IV. 



Of Adventitious Roots. 



Under the name of Adventitious Roots I desia:- 

 nate those radical processes which, instead of springing 

 from the radical trunks, are developed upon the stem 

 and branches, or sometimes upon other organs. These 

 roots (at least in Exogenous trees) proceed from the 

 Lenticels, which we have described in Book I. Chap. X. 

 Sometimes, as in Sedum altissimum, we see them spring 

 from the old cicatrices. As for Exogenous herbs, and 

 those trees of this class in which no lenticels have been 

 discovered, and also with regard to Endogens, the 

 adventitious roots may proceed from almost every part 

 of the surface, and their development is caused by the 

 lengthened contact of moisture with a part of the sur- 

 face disposed to this production of roots ; it is favoui'ed 

 by darkness, heat, and especially by a small quantity of 



