734 
DR. CARPENTER ON THE MUTUAL RELATIONS OF 
are employed for other purposes in the economy, instead of being appropriated by 
the cells alone. Thus the cells of the spongioles absorb the water, and those of the 
green surfaces obtain the carbon (from the carbonic acid of the atmosphere), which 
are required for the nutrition of the entire fabric ; and it is especially in the cells of 
the leaves that those assimilating processes are performed, whereby the plastic fluid 
is prepared, at the expense of which the organization of new tissue may take place 
elsewhere, or from which the cells in remote portions of the fabric may draw the 
materials of their peculiar secretion. So, again, we find that the process of multi- 
plication of cells is limited to certain parts in which the actions of growth are most 
actively going on ; and that a large proportion of the solid fabric is composed of 
structures which hav^e ceased to take any active share in the performance of the vital 
functions, and which retain their integrity simply because they are not exposed to 
influences that would occasion their deconjposition. 
Everywhere it is to be noticed, that if the condition of the tissues is such as to 
cause it to be changed by the play of the ordinary chemical affinities, it can only re- 
tain its normal character so long as it is performing vital actions ; and when these 
cease, it either undergoes decay (which is the case with the softer tissues), or it be- 
comes transformed into a substance which resists decay, as is seen in the conversion 
of sap-wood ” into ‘‘ heart-wood” by the filling-up of the woody tubes with sclero- 
gen, resinous secretions, &c., which have little tendency to decomposition. And it 
will be observed, too, that the combined influence of warmth, air, and moisture, which 
favours the rapid decay of dead tissue, is that which most promotes the growth of 
the living plant. Further, the more rapid and energetic are the processes of growth, 
the sooner (generally speaking) are they succeeded by decomposing changes ; this is 
seen especially in the Fungi, whose growth is more speedy, and whose degeneration 
is more immediately consequent upon the completion of their term of life, than that 
of any other tribe of plants ; and it is seen also in cases in which the leaves have 
been forced into extraordinary activity by an excess of heat and light, their death 
and exuviation being thus induced at a comparatively early period. Conversely, if 
the vital operations be retarded by the withdrawal of the external agencies on which 
they are dependent, we find that the life of the structure is proportionally prolonged, 
and the decomposing changes are retarded accordingly ; this is seen in the well- 
known fact, that a bouquet of flowers may be made to preserve its appearance of 
freshness for some time longer when kept in a dark room than it would do if exposed 
to light. These facts, and many others which might be cited, indicate that every 
integral part of the living fabric possesses within itself a capacity of being so acted 
on by external agencies, that the very forces which would tend to decompose and 
destroy it if it were dead, only excite it to vital activity if it be alive ; but that this 
capacity lasts no longer than the completion of its own term of growth, every indi- 
vidual cell being destined to pass through a certain series of changes, the completion of 
which leaves it at the mercy of the physical and chemical agencies to which it may 
