TUBEROUS ROOTS. 193 



injury to the plant, has not been ascertained. We 

 once made some experiments on the subject, but they 

 were not satisfactory, as in the case of those plants 

 that were killed by it, we could not say how much of 

 the injury was done by the actual privation, and how 

 much by the way in which that was performed. There 

 is one circumstance worthy of notice, however, and 

 that is, that the development of the gem brings on 

 some sort of general action in the old tuber. When 

 the tuber is in a state of inaction, no wound in it is 

 mortal to any of the progeny that it could produce, if 

 it do not divide or otherwise mechanically injure some 

 of the gems, and that wound remains without any kind 

 of cicatrix, and merely dries a little by the action of 

 the air. When, however, the tuber is in the earth, and 

 the gem has began to act, the wounds skin over, not 

 with an epidermis like the natural surface, but with a 

 gummy pellicle which preserves the juices from being 

 wasted in any other way than by the umbilical cord, 

 and hinders the damp of the ground from penetrating 

 the surface. Like most other vegetable substances, 

 therefore, the tuber of the potatoe is both dead and 

 alive at the same instant; dead in all parts but the 

 gems, in which it can only be killed, and alive in them. 

 Those live parts are not, however, of any specific 

 magnitude, but are hardly visible ; and yet it is by 

 means of them, and of them only, that the whole can 

 be called into action; for if they are removed, the tuber 

 rots in the ground, without even cicatrizing in the 

 wounded part. Thus, wherever we seek the principle 

 of vegetable life, we chase it towards a point : it is 

 always the less active in the same species and circum- 



