THE ROOTS OF VASCULAR PLANTS. 209 



the roots of House-leek (Sempervivum), Sec. are in some 

 cases exposed to the air, in others buried under ground. 

 We give a more exact idea of this organ in saying that 

 the Root (radix; racinej is that part of the plant 

 which, at its origin, tends to descend towards the centre 

 of the earth with more or less energy. It is to this 

 prevailing character of roots that some naturalists have 

 made allusion when they have designated the root, in a 

 general manner, under the name of Descensus. We 

 have already seen that the point of junction of the root 

 with the stem bears the name of the Neck ; from it the 

 root and stem proceed in opposite directions, so that 

 the part of each of these organs nearest the neck is the 

 oldest and usually the thickest part of the whole organ ; 

 it may be considered as the base, whatever be its po- 

 sition : the part of the root which is near the neck has 

 been named the Base or the Head of the root (caput 

 seu basis radicis) ; the part which is the most distant is 

 designated by the name of the Extremity of the root 

 (Caudex radicis, Bose ; Caudex descendens, Lin.) The 

 root and stem form, as we have seen, two conical or 

 cylindrical bodies, applied to one another by their 

 bases, and growing by the apices ; whence it necessarily 

 results that the ramifications of these two organs are in 

 an inverse direction one to another ; the stems divide 

 from the base upwards, and the roots from above down- 

 wards, — a difference which gives a very simple means of 

 knowing them in certain doubtful cases. 



A second character of Roots is that, excepting some- 

 times the extremity or spongiole, they do not become 

 green, even when they are exposed to the air and light, 

 the action of which almost always tends to render the 

 leaves and stems green. When we see the constant white- 

 ness of Roots, we are tempted to attribute it to the 

 subterranean situation which seems peculiar to them ; but 



VOL. I. p 



