224 ACTION OF FORESTS ON THE SURFACE-PLOW OF RAIN WATER. 



growth of trees, and at the same time too retentive of moisture to 

 part with it readily to the capillary attraction of the roots. 



" The trees, finding abundant nutriment near the surface, and so 

 sheltered against the action of the wind by each other as not to need 

 the support of deep and firmly fixed stays, send their roots but a 

 moderate distance downwards, and indeed often spread them out like 

 a horizontal network almost on the surface of the ground. 



" In the artificial wood, on the contrary, the spaces between the 

 trees are greater ; they are obliged to send their roots deeper, both 

 for mechanical support and in search of nutriment, and consequently 

 serve much more effectually as conduits for perpendicular drainage." 



If attention be given to the mode of growth in roots and rootlets, 

 it may be seen how it comes to pass that the fact is as stated by 

 Mr Marsh. 



The root does not bore its way into and through the soil it 

 enters, but it is prolonged, as is the line of dominoes by the players 

 with these. The extreme cells, and probably the extreme cells only, 

 or chiefly, multiply in the spongioles in accordance with the usual 

 mode of cell genesis by the formation of cells within cells, and that 

 in proportion as thoy are supplied with nutriment ; and if this 

 multiplication of cells occasion a pressure in all directions, it tends 

 more to the extension of the root than to what may be called the up 

 rooting of the tree. The direction of growth appears to be to whence 

 chiefly come the supplies of nutriment, or material of growth. 

 Where this is found on the level reached the roots extend. Where it 

 is found in the soil below them they descend. Where it is found in 

 the soil above them they ascend. I have had reported to me a case 

 of roots extending towards a ditch, descending by the side of this, 

 changing then their course so as to pass beneath it, ascending then 

 on the further side till they came near the surface, and then spreading 

 horizontally as at first. 



I have often heard of roots growing towards a quantity of assimilate- 

 able material as if guided by instinct ; and I am given to understand 

 that in willows by a stream there is generally a much more copious 

 growth of roots on the side towards the stream than on the other 

 sides around. Roots of forest trees are often prevented from descend- 

 ing by plantia>T; them above a flat stone, which cuts off supplies ; and 

 roots of fig-trees growing upon rooks follow and penetrate every crack 

 and crevice where, and where alone, they find nutriment. 



Thus may the downward growth of roots of planted trees be ac- 



