244 PROCESS OF DEVELOPEMENT. CHAP. IV. 



tion. The leaf changes its colour in the autumn, 

 and wood when felled not only changes its colour, 

 but becomes also more durable without the intro- 

 susception of any new substance. 



Though But although the doctrine of a circulation as 

 JfeEST 1 maintained by Mr. Knight should be felse, yet the 

 luminous. accou nt which he gives of the progress and agency 

 of the sap and proper juice, short of circulation, 

 may be true. The sum of the account is as fol- 

 lows : When the seed is deposited in the ground 

 under proper conditions, moisture is absorbed and 

 modified by the cotyledons, and conducted directly 

 to the radicle, which is by consequence first de- 

 developed. But the fluid which has been thus 

 conducted to the radicle, mingling no doubt with the 

 fluid which is now also absorbed from the soil, 

 ascends afterwards to the plumelet through the 

 medium of the tubes of the alburnum. The plume- 

 let now expands and gives the due preparation to 

 the ascending sap, returning it also in its elaborated 

 state to the tubes of the bark, through which it 

 again descends to the extremity of the root, forming 

 in its progress new bark and new alburnum ; but 

 mixing also, as Mr. Knight thinks, with the albur- 

 num of the former year, where such alburnum 

 exists, and so completing the circulation. 



But in this account of the process of vegetation, 

 though sufficiently perspicuous, one or two links of 

 the chain are obviously omitted ; no conjecture 

 being offered with regard to the origin of the tubes 



