GEOWTH BY THE BOOT. 15 



tain and general to merit quotation has yet been as- 

 certained concerning this important subject, except 

 the following facts, viz. that roots are most readily, 

 if not exclusively, formed in darkness and moderate 

 moisture ; that they are not, like branches, the de- 

 velopement of previously formed buds, but appear 

 fortuitously and irregularly from the woody rather 

 than the cellular part of a plant ; and that their pro- 

 duction is in some way connected with the presence 

 of leaves or leaf-buds, because portions of a stem 

 having neither leaves nor leaf-buds produce roots un- 

 willingly, if at all ; and that such roots perish if their 

 appearance be not speedily followed by the formation 

 of leaves. Thus, although the first appearance of the 

 root in the embryo plant, at the time of germination, 

 precedes the expansion of the seed-leaves, yet the 

 root will not live unless the seed-leaves are enabled 

 to act. 



31. But although the immediate cause of the for- 

 mation of roots is unknown, the remote cause is appa- 

 rently the elaboration of organisable matter by the 

 leaves ; for there can be no doubt that the develope- 

 ment of roots is much assisted by the descending sap. 

 When a ring of bark is removed from a branch, 

 if the wound is wrapped in damp moss, roots will 

 invariably push from the upper lip of the wound, 

 while the lower will produce none ; a fact so well 

 known, that it has been one of the causes of an opi- 

 nion, that roots are bundles of wood liberated from 

 the central perpendicular system, and that the wood 



