DECLINE OF PLANTS. 



thrive in the tropics. An excess, or a deficiency, of heat and light, are alike injurious; 

 both lead to functional derangement, ending in general debility. A gardener, on receiving 

 an exotic plant new to him, would desire to know its native country, and what soils and 

 situations it preferred in its wild state, in order that he might determine what mode of 

 culture would be most likely to be suitable to it. But different species differ widely in 

 their pov^^er of adapting themselves to different climates, and this the gardener can only 

 learn by experience, aided by his knowledge of the geographical range of the plant in ques- 

 tion. Does our knowledge, then, of the native country of the potato, and of the various 

 climates in which it is cultivated, warrant the conclusion that the curl disease is caused 

 simply by the over-ripening of the seed tubers.'' that is, (if I understand correctly what is 

 implied,) is the amount of heat and light of European summers, greater than the plant 

 naturally requires for its healthy growth? At the first glance, it seems hardly probable 

 that a plant which is a native of the tropical regions of South America, should be over- 

 ripened in the climate of England. Of all cultivated plants, the potato is most accom- 

 modating. It is cultivated in every latitude from the torrid to the frigid zone, and if it is 

 liable to be so over-ripened in England, as to cause it to be diseased, then what should be the 

 condition of the plant when grown in the West India Islands; in the burning sands of 

 the Cape, or under the hot and brilliant summers of the United States? The quantity and 

 quality of the secretions of a given plant, and the solidity of its tissue, depends partly on 

 the amount of light and heat to which it is exposed, and if the concrete state of the sap, 

 or the dry condition of the tuber of the potato, which gives rise to curl, was simply or 

 solely the effect of over-ripening, in England, it should follow that the plant would be use- 

 less in the United States of America, and the West Indies, or at the Cape. 



I am not aware that any satisfactory evidence exists, to show that the ripe state of the 

 tuber of an aged variety induces a certain condition of the plant, which is favorable to the 

 attack of some animal or vegetable parasite; but there are facts on record which seem to 

 justify the conclusion that a change does take place in the tubers of a given variety, when 

 in the course of time its vigor declines, the tissue becomes more solid and drier, and the 

 fluid thicker and less plentiful; or in other words, the tubers of a given variety become 

 more dry and farinaceous. 



In animals, it is well known that certain changes do take place as age creeps on. The 

 bones gradually become more solid and brittle; the muscles more rigid, and the fluids 

 thicker and less abundant; and various important organs, on the due exercise of which 

 health depends, become impaired, and incapable of performing efficiently their respective 

 functions. The opponents of Mr. Knight's theory have dwelt much upon this fact, when 

 doubting the accuracy of the conclusions at which he had arrived respecting the limited 

 duration of individual plants. An animal, say they, becomes worn out, or dies, when old, 

 in consequence of a structural change in many of its most important organs; but in nlants 

 or trees there is nothing analogous to this. I suspect, however, that there is a closer anal- 

 ogy between plants and animals in this respect, than has hitherto been supposed to exist. 



The life of an animal is marked by three distinct stages, progressive, conservative, and 

 declining. In youth, the greatest amount of food is assimilated; the body increases ra- 

 piply in size, and the limbs are supple; in middle age, little more food is appropriated 

 than is required for the repair, or solidifying of the frame; while in the decline of life, an 

 animal gradually becomes meagre and diminishes in size — the quantity of food assimilated 

 is not equal to the waste of the body. " It is certain that the productive powers of a va 

 of the potato, is in proportion to its youth." (Lindlet.) 

 certain, that owing to a progressive decline of the vital powers, possibly to the 



