THE ROOTS OF VASCULAR PLANTS. 215 



the descending juices ; and according as these nodes are 

 exposed either to the shade, drought, or moisture, they 

 produce branches or roots. It is for tliis reason that 

 stems naturally knotty are more easily multiplied by 

 layers or cuttings than others, as is seen in the Pink, 

 the Vine, &c. When the cellular tissue of the bark of 

 stems is very abundant or fleshy, the bark itself is con- 

 stantly moist, and the juices are more stagnant there. 

 Plants which present these characters have hkewise a 

 predisposition to shoot out roots, even when fully ex- 

 posed to the air, as is seen in succulents, particularly in 

 Cactus-, Crassula, Sedum, Sec. It is the same with 

 roots : the tubercules which are developed upon some of 

 them are kinds of stores or magazines of juice; these 

 likewise have a singular tendency to emit new produc- 

 tions. 



These facts, well known to all cultivators, have sug- 

 gested to Duhamel and some others the idea of a bold 

 experiment, from which false conclusions have often 

 been drawn. A tree is selected which easily takes root 

 by layers, such as the Willow ; its top is bent towards 

 the ground ; the extremities of its branches are fastened 

 down, and soon strike root there : when the roots are 

 developed, the trunk of the tree is raised up so as to 

 expose its old roots to the air, and place it in an inverted 

 position ; after some time it forms a new cyme furnished 

 with leaves and branches. Mustel and some physiolo- 

 gists, who have spoken of this experiment of inverting 

 trees, say that the branches are changed into roots, and 

 the roots into branches, and they quote this fact as a 

 decisive proof of the identity of these two organs ; but 

 this experiment, when better analysed, tends, on the 

 contrary, to show their difference. 



It is true that the branches here shoot out roots, but 

 all the young shoots perish when they are put into the 



