98 



LETTERS ON TREES. 



it for a little, and see where it exactly lies, and how it 

 acts. The old rootlets and their spongioles have, I 

 apprehend, no real concern in it, but only the new — 

 those that have grown during the year, which stand 

 related to the new plants, and to the corresponding 

 layer of woody tissue, and which, as we shall yet see, 

 actually begin to grow contemporaneously with these.^ 

 It may be supposed, however, to be of the nature of 

 an a tergo force — that is, of a moving power acting 

 from behind and in the way of propulsion — differing, 

 therefore, in its mode of action, if not in its essential 

 nature, from that already considered. It is clearly, 

 as the facts connected with it show, elective in the first 

 instance — allowing certain fluid matters existing in the 

 soil to gain entrance within the tree, and rejecting 

 others.^ But in the next place, with regard to the 

 fluid thus admitted, it is as clearly attractive as is the 

 agency of the buds and of the living structures which 

 spring from them. So soon, indeed, as it is drawn 



* See Letter XII., Sections 7-13. 



f " If a grain of wheat and a pea be grown in the same soil, the 

 former will obtain for itself all the silex, or flinty matter, which the 

 water can dissolve ; and it is the deposition of this in the stem which 

 gives to all the grasses so much firmness. On the other hand, the 

 pea v^ill reject this, and will take up whatever calcareous substances 

 (or those formed of lime and its compounds) the water of the soil 

 contains, these being rejected by the wheat." — Dr Carpenter. This 

 selecting power, however, is limited, as in the case of the animal 

 body — substances being readily absorbed which prove hurtful to the 

 system, or even fatal. 



