Now why is this? Owing to a difference in constitutional vigor, certainly; but do not these plants afford indications of a progressive alteration in the abundance and viscidity of the fluids?

The tuber of the potato is in its structure, analogous to a branch; it is, physiologically speaking, an under-ground stem. And the tubers of a given variety are just as much the extension of an individual potato plant, as the cuttings or grafts of a variety of fruit, are the extension of an individual tree. Now we have seen that the tubers of a variety of potato just obtained from seed, contain a greater amount of disposable fluid sap than they do at any other period of the existence of the same variety. The plants are then the most luxuriant, and the produce of tubers, or under-ground stems, the most abundant. So of fruit and other trees. For many years after an apple tree has sprung from seed, the young plant or plants raised from cuttings or grafts of it, show no disposition to bear fruit. Why? The vigorous growth of strong, sappy, elastic shoots, and the abundant and large foliage, afford the answer; they indicate an abundant flow of sub-aqueous sap, which is opposed to fruitfulness. After a time the luxuriant growth of branches gradually abates, and the tree as gradually becomes more fruitful. So of the variety of potato. When the luxuriance of youth has subsided, the under-ground stems gradually decrease in quantity, but increase in quality or dryness.

Now what are the conditions required for the production of fruit by a tree; are they not a moderate degree of growth, and a store of highly elaborated, or concrete sap? If, then, the plants of a fruit tree gradually become more productive as the parent plant advances towards the prime of its existence, is it not evident that the sap, under ordinary circumstances, must become more highly elaborated, and stored in the tissue in a more dry and concrete state than in youth, exactly as in the case of the potato. What an evident similarity subsists between the tree and the vegetable, on the approach of old age. The tubers of the potato continue to diminish in quantity; they now also lose their quality, till the plant becomes utterly useless. So the branches of the declining tree become gradually more meagre, and fruit is sparingly, and less frequently produced; thereby indicating that in the branches of the aged tree, as well as in the tubers of the aged potato plant, there must be a progressive diminution in the quantity of the secretions deposited.

How admirably this conclusion is borne out by an observation by Mr. Knight; " the wood of all the old fruit trees," said he, " has long appeared to me to possess less elasticity and hardness, and to feel more soft and spongy under the the knife, than that of new varieties. This defect may, I think, be the immediate cause of the canker and moss, though it is probably, itself, the effect of old age, and therefore incurable".

If I may hazard an opinion as to the cause of these progressive changes, I would suggest whether they are not simply owing to a progressive diminution in the activity of the absorbent vessels. We see that the vigor of the growth of a tree, and of a variety of potato, is in proportion to their youth; may not, therefore, the rapidity of the circulation of the fluids, as well as their abundance, diminish as the tree or potato becomes aged. If there is more energetic absorption by the roots in youth, may not the sap be propelled with greater force, under ordinary circumstances, than in mature, or old age, and thus circulate with greater rapidity, through the leaves; therefore be less elaborated, retain more of a fluid character, and be unsuited for the formation of blossom buds, or the support of fruit, though admirably adapted for the formation of fresh branches and roots. May not, also, the gradual diminution of the growth of branches and tubers, and the cor* responding increase of the density of secretions, as the tree or potato approaches the prime of life, be owing to a more moderate yet more liberal supply, and to a less rapid flow of the sap; in consequence of which it passes more deliberately through the leaves, becomes more highly elaborated, and therefore better suited to the production of blossoms and fruit, than in its youth.

The increase of the roots of a tree is commensurate with the increase of its branches; if we head-back the branches of a healthy tree in full bearing, it immediately produces strong, sappy shoots, because the absorbent power of the roots is equivalent to the wants of a greater breadth of foliage, and little or no fruit is produced till the balance between branches and roots is nearly restored. Thus a similar effect is produced, as by the smaller system of roots, but more energetic absorbent power of a tree newly raised from seed. If, on the other hand, we allow the branches of a vigorous tree to remain, and adopt means to diminish its roots, as by the practice of root-pruning, we diminish the supply of sap, diminish the growth of branches, and make the tree more fruitful.

I desire further to suggest, whether it is not probable, that not only must there be a progressive diminution in the absorbent power of a plant as it becomes aged, but whether its annual efforts must not be less sustained in age, than in youth. Early in the season tissue is most rapidly developed; as the season advances it is perfected, or solidified. According to the brightness of the light, or as the season is favorable to the exertions of the plant, all other circumstances being similar, so is the density of its secretions, or the amount of matter which it is capable of depositing in previously formed cells. Now, do not the soft spongy tubers of a nearly expended variety of the potato, and the spongy branches, and irregularly ripened fruit of an aged tree, indicate that the exertions of the plant must become so feeble as autumn approaches, that it is not able to absorb and elaborate a sufficient quantity of sap to perfect the tissue, or feed the whole of the fruit which it formed in spring.

This view of the effects of age on plants, I hope to be able to investigate experimentally 5 in the mean time, I am desirous that the thoughts which hare occurred to me on the sub-ject, should find a place in your Journal. Respectfully yours. JOHN TOWNLEY.

Moundville, Marquetts County,Wis.,Dec.30.