Horticultural Chemistry. 133 



whenever occasion requires, are so porous as at all times 

 freely to admit the pervasion of the atmosphere ; and there- 

 fore, by this extra-exposure, the vegetable and animal remains 

 are hastened in decomposing, and much of their fertile con- 

 stituents evolved in the state of gas, or carried away by the 

 rains, &c, without there being any crop upon them to benefit 

 by them. Thus Theory argues, and Practice certainly seems 

 to support in this instance her doctrines. Switzer, one of our 

 horticultural classics, says : " Rich heavy ground cannot well 

 be ploughed too often to make it light, and the better manure 

 by killing the weeds ; as light poor ground cannot be ploughed 

 too seldom, for fear of impoverishing it." [Ichnographia Rus- 

 tica, vol. iii. p. 237.) 



We have seen that plants search after and acquire food by 

 the agency of their roots; and the extremities of these appear 

 to be the chief if not only parts employed in the intro-sus- 

 ception of all food not in a gaseous state, for M. Duhamel 

 observed that that portion of a soil was soonest exhausted in 

 which the greatest number of the extremities of the roots 

 were assembled. [Physique des Arbres, vol. iii. p. 276.) This 

 explains why the fibrous points of roots are annually renewed, 

 and the caudex extended in length ; by these means they each 

 year shoot forth into a fresh soil, always changing their direc- 

 tion to where most food is to be obtained. If the extremity 

 of a root is cut off it ceases to increase in length, but enlarges 

 its circle of extension by lateral shoots. It is by their extre- 

 mities, then, that roots imbibe food ; but the orifices of these 

 are so minute, that they can only admit such as is in a state of 

 solution. Carbon, reduced to an impalpable powder, being 

 insoluble in water, though offered to the roots of several 

 plants, mingled with that fluid, has never been observed to be 

 absorbed by them ; yet it is one of their chief constituents, 

 and is readily absorbed in any combination which renders it 

 fluid. 



Roots, then, obtain such nourishment to plants from a soil 

 as is in a gaseous or liquid state; we may next, therefore, con- 

 sider what constituents of soils are capable of being presented 

 in such forms. Water can be the only solvent employed ; 

 indeed, so essential is this liquid itself that no plant can exist 

 where it is entirely absent, and on the other hand many will 

 exist with their roots in vessels containing nothing but dis- 

 tilled water. Plants with a broad surface of leaves, as mint, 

 beans, &c, I have always found increase in carbonaceous mat- 

 ter, whilst thus vegetating; but onions, hyacinths, &c, with 

 small surfaces of foliage, I as invariably have found to decrease 



k 3 



