236 PHYSIOLOGY. BOOK II. 



the temperature of 3^^, and screened from the action of light, 

 its integument gradually imbibes moisture and swells, the 

 tissue is softened, and acquires the capability of stretching, 

 oxygen is absorbed, carbonic acid 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 penetrating the soil, the latter elevat- 

 ing themselves above it, acquiring a green colour by the 

 deposition of the carbon they absorb from the atmosphere, and 

 unfolding in the form of two opposite roundish leaves. Tills 

 is the first stage of vegetation : the young plant consists of 

 little more than cellular tissue ; only an imperfect develope- 

 ment of vascular 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 prepara- 

 tion for the medullary sheath, and consists of vertical fibres 

 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 of the pith, and 

 is thence impelled into the cotyledons, where it is aerated 

 and evaporated, and urged upwards against the growing 

 point or plumule : such of it as is not fixed in the cotyledons 

 passes down through the bark into the root. 



Forced onwards by the current of sap, which is continually 

 impelled upwards from the root, the plumule next ascends in 

 the form of a little twig, at the same time sending roots 

 downwards in the centre of the radicle, in the form of fibres, 

 which become the earliest portion of wood that is deposited : 

 these fibres, by their action, now compel the root to emit 

 little ramifications. 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, imtil, 

 when the first internode of the latter ceases to lengthen, the 

 leaf has actually ai'rived at its complete formation. When 

 fully grown it repeats in a much more perfect manner the 

 functions previously performed by the cotyledons : it aerates 



