FUNCTION.] GENERAL CONSIDERATIONS. 133 



with the carbon of the seed, forms carbonic acid, which is 

 expelled ; nutritious food for the young parts is prepared by 

 the conversion of starch into sugar ; and the vital action of 

 the embryo commences. It lengthens downwards by the 

 radicle, and upwards by the cotyledons ; the former penetra- 

 ting the soil, the latter elevating themselves above it, acquir- 

 ing a green colour by the decomposition of the carbonic acid 

 they absorb from the earth and atmosphere, and unfolding in 

 the form of two opposite roundish leaves. This is the first 

 stage of vegetation ; the young plant consists of little more 

 than cellular tissue ; only an imperfect development of vascu- 

 lar and fibrous tissue being discoverable, in the form of a sort 

 of cylinder, lying just in the centre. The part within the 

 cylinder, at its upper end, is now the pith, without it the 

 bark; while the cylinder itself is the preparation for the 

 medullary sheath, and consists of vertical tubes passing 

 through and separated by cellular tissue. 



The young root is now lengthening at its point, and 

 absorbing from the earth its nutriment, which passes up to 

 the summit of the plant by the cellular substance, and is, in 

 part, impelled into the cotyledons, where it is aerated and 

 evaporated, but chiefly urged upwards against the growing 

 point or plumule. 



II. Forced onwards by the current of sap, which is con- 

 tinually impelled upwards from the root, the plumule next 

 ascends in the form of a little twig, at the same time sending 

 downwards, in the centre of the radicle, the earliest portion of 

 wood that is deposited, and compelling the root to emit little 

 ramifications ; and simultaneously the process of lignification 

 is going on in all the tissue, by the deposit of a peculiar 

 secretion in layers within the cells and tubes. 



Previously to the elongation of the plumule, its point has 

 acquired the rudimentary state of a leaf : this latter continues 

 to develope as the plumule elongates, until,- when the first 

 internode of the latter ceases to lengthen, the leaf has actually 

 arrived at its complete formation. When fully grown it re- 

 peats in a much more perfect manner the functions previously 

 performed by the cotyledons : it aerates the sap that it 



