Book I. NATURAL DECAY OF VEGETABLES. 563 



to be attempted by general culture, and particular attention to hinder the propagation of the Insects Or 

 vermin, whether oviparous or otherwise, by destroying their embryo progeny. 



1710. Sometimes the disease is occasioned by an extravasation of juices which coagulate on the surface 

 of the stalk, so as to form a sort of crust, investing it as a sheath,-and preventing its farther expansion. 



1711. Sometimes the disease arises /rom want of an adequate supply of nourishment as derived from the 

 soil, in which case the lower part of the plant is the best supplied, while the upper part of it is starved. Hence 

 the top shoots decrease in size every succeeding year, because a sufficient supply of sap cannot be obtained 

 to give them their proper developement. This is analogous to the phenomena of animal hfe, when the 

 action of the heart is too feeble to propel the blood through the whole of the system : for then the 

 extremities are always the first to suffer. And perhaps it may account also for the fact, that in bad soils, 

 and unfavourable seasons, when the ear of barley is not wholly perfected, yet a few of the lower grains 

 are always completely developed. [Smith's Introduction, p. 279.) 



1712. Contortion. The leaves of plants are often injured by means of the puncture of 

 insects, so as to induce a sort of disease which discovers itself in the contortion or convo- 

 lution of the margin, or wrinkled appearance of the surface. The leaves of the apricot, 

 peach, and nectarine, are extremely liable to be thus affected in the months of June and 

 July. The leaves of the apple are affected by the A'phis lanigera ; those of the larch by 

 another woolly aphis (A. laricio) ; those of the hawthorn by a species of Tenthredo, &c. 

 {See Major s Treatise on the Insects prevalent in Fruit Trees and Garden Produce.) 



1713. The leafwYnch has been punctured soon begins to assume a rough and wrinkled figure, and a red. 

 dish and scrofulous appearance, particularly on the upper surface. The margins roll inwards on the under 

 side, and enclose the eggs which are scattered irregularly on the surface, giving it a blackish and granular 

 appearance, but without materially injuring its health. In the vine, the substance deposited on the leaf 

 is whitish, giving the under surface a sort of a frosted appearance, but not occasioning the red and scrofu- 

 lous aspect of the upper surface of the leaf of the nectarine. In the poplar, the eggs when first deposited 

 resemble a number of small and hoary vesicles containing a sort of clear and colourless fluid. The leaf 

 then becomes reflected and conduplicated, enclosing the eggs, and exhibiting a few reddish protuberances 

 on the upper surface. The embryo is nourished by this fluid ; and the hoariness is converted into a fine 

 cottony down, which for some time envelopes the young fly. The leaf of the lime tree in particular, when 

 fully expanded, is hable to attacks from insects ; and hence the gnawed appearance it so often displays. 

 The injury seems to be occasioned by some species of puceron depositing its eggs in the parenchyma, 

 generally about the angles that branch off" from the midrib. A sort of down is produced, at first green, and 

 afterwards hoary; sometimes in patches, and sometimes pervading the whole leaf; as in the case of the 

 vine. Under this covering the egg is hatched ; and then the young insect gnaws and injures the leaf, 

 leaving a hole or scar of a burnt or singed appearance. Sometimes the upper surface of the leaf is covered 

 with clusters of wart-like substances somewhat subulate and acute. They seem to be occasioned by means 

 of punctures made in the under surface, on which a number of openings are discoverable, penetrating 

 into the warts, which are hollow and villous within. The disease admits of palliation by watering 

 frequently over the leaves ; and by removing such as are the most contorted and covered by larvae. 



1714. Consumption. From barren or improper soil, unfavourable climes, careless 

 planting, or exhaustion from too frequent flowering, it often happens that disease is 

 induced which terminates in a gradual decline and wasting away of the plant, till at 

 length it is wholly dried up. Sometimes it is also occasioned by excessive drought, or 

 by dust lodging on the leaves, or by fumes issuing from neighbouring manufactories, or 

 by the attacks of insects. 



1715. There is a consumptive affection frequently attacking the pine tree ( Willdenow, Princ. Bot. p. 351.), 

 which affects the alburnum and inner bark chiefly, and seems to proceed from long-continued drought, or 

 from frost suddenly succeeding mild or warm weather, or from heavy winds. The leaves assume a tinge of 

 yellow, bordering upon red. A great number of small drops of resin, of a putrid odour, exude from the 

 middle of the boughs. The bark exfoliates, and the alburnum presents a livid appearance : the tree swarms 

 with insects ^Dypterygia pinastri Steph.), and the disease is incurable, inducing inevitably the total 

 decay and death of the individual. The preventive is obviously good culture, so as to maintain vigorous 

 health : palliatives may be employed, according to the apparent cause of the disease. 



Sect. III. Natural Decay. 



1716. Although a plant should not suffer from the influence of accidental injury, or 

 from disease, still there will come a time when its several organs will begin to experience 

 the approaches of a natural decline insGnslhly stealing upon it, did at last inducing death. 

 The duration of vegetable existence is very different in different species. Yet in the 

 vegetable, as well as in the animal kingdom, there is a term or limit set, beyond which 

 the individual cannot pass. Some plants are annuals, and last for one season only, 

 springing up suddenly from seed, attaining rapidly to maturity, producing and sowing 

 their seeds, and afterwards immediately perishing. Such is the character of the various 

 species of corn, as exemplified in oats, wheat, and bailey. Some plants continue to live 

 for a period of two years, and are therefore called biennials, springing up the first year 

 from seed, and producing roots and leaves, but no fruit ; and in the second year pro- 

 ducing both flower and fruit, as exemplified in the carrot, parsnep, and caraway. Other 

 plants are perennials, that is, lasting for many years ; of which some are called under- 

 shrubs, and die down to the root every year ; others are called shrubs, and are perma- 

 nent both by the root and stem, but do not attain to a great height or great age ; others 

 are called trees, and are not only permanent by both root and stem, but attain to a great 

 size, and live to a great age. But even of plants that are woody and perennial, there 

 are parts which perish annually, or which are at least annually separated from the indi- 

 vidual ; namely, the leaves, flowers, and fruit, leaving nothing behind but the bare 

 caudex, which submits in its turn to the ravages of time, and ultimately to death. 



1717. The decay of the temporary organs, which takes place annually, is a phenomenon 



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