bark or albumum of either the stem or the root, he proceeded to in-
vestigate the states of the leaf, and of the succulent annual shoot.
With this view various grafts of the golden pippin, which were known
to be liable to decay, were inoculated with buds of new varieties;
and in the ensuing winter their own natural buds were removed, and
those that had been inserted were alone allowed to remain. As soon
as the leaves of these began to appear, every symptom of disease was
removed; and each part of the branch of the golden pippin thus re-
generated, appeared to perform its ofiice as well as the wood and
bark of the seedling stock could have done without this intermediate
graft of old materials.
Since the vigour of youth, or debility and diseases of old age, ap- peared thus to depend on the quality of the leaf through which the sap of plants circulates, in the same manner as the blood of animals does through their lungs, Mr. Knight considers the consequence of defective leaves, according to his former views of the functions they perform, of preparing and assimilating the sap transmitted through them; and he observes, that the deficiency of power in the leaves is (as might be expected) most apparent where the redundancy of sap is the greatest; for he finds that the grafts of old varieties are most diseased in rich soils, or when they are applied to vigorous stocks; and the defects appear to arise from an accumulation of fluid in the extreme branches and annual shoots, beyond what can circulate with effect through the imperfect leaves that are produced by extremities debilitated by age.
In support of this opinion, of an essential difference between the leaves of young and of old varieties, Mr. Knight observes, that there is an evident alteration in the character of leaves visible in the same variety, between those of two years and those of twenty years old; and that it is consequently highly probable that still further changes have occurred in the course of two centuries.
From these results respecting the importance of the leaves to the well-being of vegetables, the author ventures to suggest the proba- bility, that the debilities of old age in animals may arise from a si- milar source, and may be traced to injury primarily sustained by the lungs.
It is not merely upon general analogy that such an opinion may be supported, but in particular instances of long life in men and in domesticated animals, it is observed that those individuals longest retain their health, and are most able to bear excessive labour with- out injury to their constitution, in whom the chest is manifestly most capacious.
Since the organs of digestion in those quadrupeds which live wholly upon grass differ considerably in their construction from those
