LETTER X. 



93 



ence, it must be shewn that they contribute actually 

 and actively toAvards the movement ; and that, too. in 

 a way not referable to their porosity merely, or to any 

 other simply physical property they may possess. 

 There is a familiar experiment which you have your- 

 selves often made for your amusement — that of strew- 

 ing cress or mustard seeds on a bottle or other vessel 

 covered with flannel and placed in a shallow dish, which 

 you fill up, and from time to time replenish with water ; 

 the result being, as you know, that in due time the 

 seeds germinate and cover the vessel with living 

 plants. Here, in this simple experiment, the flannel 

 by reason of its porosity conveys, or rather allows the 

 passage of the water from the dish to the living seeds 

 and plants. No one, however, would for a moment 

 imagine that the flannel is alive. jSTo more are we 

 entitled to infer that the old stems and roots of a tree 

 are alive because of the sap moving through them to 

 the living and growing parts above. They may be 

 merely the medium or channel of its transmission,, and 

 may contribute towards this in the same way that the 

 flannel does, and in no other. Their vitahty, there- 

 fore, if they be really possessed of any, must be estab- 

 lished on other grounds than this. 



3. Unquestionably, the circulation of the sap is a vital 

 action, and due to vital agency. This agency, how- 

 ever, has its seat or centre in the hving buds, and in 

 the living structures proceeding from them, and actually 



