ON THE DESCENT OF THE SAP. 
183 
it contained in embryo, than in supposing that a certain membrane 
imperceptible to the eye should possess the power of becoming in 
succession a pulpy, gelatinous mass ; then imparting to this mass the 
properties of organisation, dividing it into liber and alburnum ; and, 
finally, peacefully taking up its abode between them unchanged or 
self-produced, ready to perform the same operation in the following 
vear. 
I fully agree with you, that nothing is produced without its seed or 
propago, but I conceive there is a difference in treating of a plant being 
produced from a seed, and holding the necessity that every expansion 
of that plant should take place in a manner exactly analogous. Grant 
that vegetation is regulated by a principle of life, and it becomes as 
much the province of that principle to make an addition annually to 
the size of a ligneous plant, as it is the province of life in the animal 
economy to enlarge the size of the members of the body, and change 
their constituent parts, through means of the nutrition or food which it 
has received and digested. Almost every gardener is conversant with 
the fact, that the vital principle is peculiarly active in the inner layer 
of liber, but he will not trouble himself much respecting your indusium, 
if the knowledge of its modes of operation will not enable him to 
perform better his different operations. I suppose you would at once 
admit the descent of the sap, if the tumour above an annular incision 
were found to contain a simple bland fluid instead of cambium or 
alburnum; but if such were the fact, I should consider it as a proof 
that the vital principle was unable to appropriate the food at its disposal 
to the purpose of additional growth. The fact to which you refer, 
that, in the case of wounds, the sap which is exuded becomes not 
cambium, but resin, gum, canker, &c., is no proof that it is impossible 
the sap should be changed into wood through processes applied to it 
when circulating in the tubes of an organised being. The appearance 
which the sap presents when exposed to the atmosphere but proves the 
existence of disease and the infliction of injury; and to expect in such 
cases the formation of wood, although in other circumstances it might 
have become so, would be as inconsistent as to expect to see the blood, 
flowing from a wound in our own body, becoming converted into bone, 
muscle, and flesh, because aware that that fluid is the agent employed, 
by the principle of life, not merely for enlarging these different sub¬ 
stances, but continually changing them in their constituent parts. I 
grant you that we cannot account for the change of sap into alburnum 
upon any chemical principle, any more than we can account for the 
mutation of blood into bone; and I would at once deny with you the 
possibility of its conversion, did you convince me that chemistry 
