DECLINE OF PLANTS. 



in the earlier years of its existence may be healthy, and then become liable to the curl. We 

 may also understand why some varieties of the potato ma}' be over-ripened in England, 

 and yet the plant be cultivated in the tropics; Avhy cold, vv'et, and cloudy seasons, adverse 

 to the growth of the potato, may produce tubers which afford the most healthy plants; 

 and why a comparatively young variety may be subject to the curl, while another variety 

 may become infirm and useless without exhibiting any marked symptoms of the disease. 

 The utility of preventing the perfect ripening of the seed-tubers of dry and farinaceous 

 varieties, is apparent. Holt observed that " the finer kinds sooner degenerate than the 

 coarse kinds, which are almost, if not alwaj'S, the most productive, and retain their vigor 

 the longest." The cause of this, too, must be now obvious. The dry and fixrinaceous tu- 

 ber, as Mr. Knight observed, " indicates some degree of approximation to disease;" an 

 observation evidently well founded. 



The changes induced in the character of the potato by age, seem calculated to throw 

 fresh light on the gradual deterioration or wearing out of trees. Trees afford, on conside- 

 ration, the same evidence as potatoes, of progressive changes, leading to functional derange- 

 ment and debility. We see in a variety of fruit trees, the vigor of youth, the productive- 

 ness of maturity, and the decrij)itude of age. These stages in the progress of life, are 

 distinctly marked. The action of external influences cannot account for them. We may 

 take three stocks of equal vigor, and graft on one a scion from a healthy tree just sprung 

 from seed; on another a scion from a tree in the middle of life, and on the third a scion 

 from an aged, almost worn out variety. Notwithstanding the equality of the stocks, the 

 trees which spring from them will exhibit unequal degrees of vigor. One will grow with 

 great luxuriance, and for some years show no disposition to bear fruit; the second will 

 grow moderately, and soon bear fruit abundantly; whilst the third will shortly manifest 

 all the symptoms of a decrepid old tree. Now wh}'- is this? Owing to a difference in con- 

 stitutional vigor, certainly; but do not these plants afford indications of a progressive al- 

 teration 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 po- 

 tato 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 qnantit}'', 

 but increase in quality or dryness. Now what are the conditions required for the produc- 

 tion 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 pro- 

 ductive 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 

 tissue in a more dry and concrete state than in youth, exactly as in the case 

 What an evident similarity subsists between the tree and the vegetable, 



