90 Mr. Burnett on the Development 



trees which have been entirely surrounded with the embank- 

 ment have wholly died, whilst those which stand on the slope 

 of the artificial mounds, so that the roots of one side only have 

 been deeply buried, have suffered much less from the cause 

 which has killed the others, though all were equally surrounded 

 with cylinders of brick, which is at least a useless, if not an 

 injurious plan. It is a common error to suppose that the 

 roots of trees pierce the earth very deeply, whereas four or five 

 feet will be often found more than the depth of the roots of 

 trees of from sixty to eighty feet in height. Roots spread much 

 further laterally, that they may gain easier access to the air, 

 and even some send up shoots at intervals to the surface, 

 apparently as respiratory organs ; and it is well known how 

 favourable the loosening of the soil is to the health of trees, as 

 well as of all other plants. 



The nutrient system of vegetables consists of several sub- 

 ordinate parts, which, like the prime organisms, are, in dif- 

 ferent instances, more or less distinct or blended with each 

 other : these are the transpiratory, the assimilating, and the 

 respiratory systems ; the transpiratory consisting of the ab- 

 sorbent and exhalant organs, the assimilating of the secretory 

 and excretory, and the respiratory perhaps of the inspiring 

 and expiring ones, or at least performing functions equivalent 

 thereto. Concerning all these much contrariety of opinion 

 both has existed and does still exist : on the one hand, (to 

 take two extreme examples,) it is contended that the root is 

 the sole organ of absorption, and that on this point the leaves 

 are wholly inefficient ; while, on the other, it is as strenuously 

 contended that the leaves are the chief, if not the sole absorb- 

 ents, and that the root serves merely as a fulcrum to steady 

 the plant and attach it to the soil in which it stands ; the one 

 maintaining the ascent, the other the descent of the sap. 



Again, with regard to the channels through which the 

 transit is performed, opinions as contrary exist : one doctrine 

 being that it ascends by means of the spiral vessels, another 

 that it passes through the non-spiral tubes, and a third de- 

 clares that, although the plant may be composed of cells and 

 tubes, the sap mounts not in them, but is transmitted from 

 one part of the vegetable to another, by choosing for its pas- 



