250 PHYSIOLOGY OF TREES. 



distends, connects, and invigorates the whole 

 system. It is increased by supplies imbibed by 

 syphon-acting tubes from the earth, and also by 

 absorption of moisture from the air. It is found 

 of many different qualities in different plants ; 

 each kind of plant possessing peculiar powers 

 and structure for elaborating, concocting, and 

 assimilating the qualities of the earth and air, so 

 as to form specific combinations proper to itself. 

 Thus the Pi mis genus possesses an organisation 

 and a fountain for the production of resin ; Mi- 

 mosa^ gum ; RicinuSy oil ; &c. Other plants ela- 

 borate sugar, starch, &c. ; and though these 

 specific qualities are often more concentrated in 

 one part than in others, yet the whole plant par- 

 takes of them in a greater or lesser degree. 



Sap is of various consistency ; in some plants 

 it is like pure water, without colour, scent, or 

 taste. It is, on the contrary, in some coniferous 

 plants so inspissated, and (whether left in a 

 cavity of the timber, or withdrawn by tapping) 

 becomes so concreted that it resists edge-tools, 

 and so indurated as to last for ages, unless de- 



