262 SCIENCE OF AGRICULTURE. Part II. 



look always peculiarly pale. But if they are removed from such situatfomr, and exposed 

 to the action of light, they will again recover their green color. Etiolation may also en- 

 sue from the depredation of insects nestling in the radicle, and consuming the food of the 

 plant, and thus debilitating the vessels of the leaf so as to render them insusceptible of 

 the action of light. This is said to be often the case with the radicles of secale cereale ; 

 and the same result may also arise from poverty of soil. 



1672. Suffocation. Sometimes it happens that the pores of the epideripis are closed up, 

 and transpiration consequently obstructed, by means of some extraneous substance that 

 attaches itself to and covers the bark. This obstruction induces disease, and the disease 

 is called suffocation. 



1673. Sometimes it is occasioned by the immoderate growth of lichens upon the bark covering the whole 

 of the plant, as may be often seen in fruit-trees, which it is necessary to keep clean by means of scraping 

 off" the lichens, at least from the smaller branches. For if the young branches are thus coated, so as that 

 the bark cannot perfoim its proper functions, the tree will soon begin to languish, and will finally become 

 covered with fungi, inducing or resulting from decay, till it is at last wholly choked up. 



1674. But a similar effect is also occasionally produced by insects, in feeding upon the sap or shoot. This 

 may be exemplified in the case of the aphides, which sometimes breed or settle upon the tender shoot in 

 such multitudes as to cover it from the action of the external air altogether. It may be exemplified also 

 jn the case of Coccus Hesperidum and Acarus tetlarius, insects that infest hot-house plants, the latter by 

 spinning a fine and delicate web over the leaf, and thus preventing the access of atmospheric air. Insects 

 are to be removed either by the hand or other mechanical means, or destroyed by excess of some of the 

 elements of their nutrition, as heat, or cold, or moisture, where such excess does not prove injurious to 

 the plant ; or by a composition either fluid or otherwise, which shall have the same effects. Prevention 

 is to be attempted by general culture, and particular attention to prevent the propagation of the insects or 

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



1675. 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 further expansion. 



1676. Sometimes the disease is occasioned fro7n want of an adequate supply of nowishment as derived 

 from the soil, in which 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 sufficient supply of sap cannot be 

 obtained to give them their proper developement. This is analogous to the phenomena of animal life, 

 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 unfavorable 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. 344.) 



1677. Contortion. The leaves of plants are often injured by means of the puncture of 

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

 tion 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. 



1678. The leaf that has been punctured soon begins to assume a rough and wrinkled figure, and a reddish 

 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 scrofulous 

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

 semble a number of small and hoary vesicles containing a sort of clear and colorless fluid. The leaf then 

 becomes reflected and conduplicated, enclosing the eggs, with 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 is liable to attacks 

 from insects when fully expanded ; and hence the gnawed appearance it so often exhibits. 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 greeji, 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 a puncture 

 made on 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 larvse. 



1679. Consumption. From barren or improper soil, unfavorable climes, careless 

 planting, or too frequent flowering exhausting the strength of the plant, 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 manufactories which 

 may happen to be situated in the neighborhood, or by the attacks of insects. 



1680. There is a consumptive affection that frequently attacks the pine-tree, called Teredo Pinorum 

 (Wildenow, Princ. Bat. 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 heavy winds. 

 The leaves assume a tinge of yellow, bordering upon red. A great number of small drops of resin exude 

 from the middle of the boughs, of a putrid odor. The bark exfoliates, and the alburnum presents a livid 

 appearance. The tree swarms with insects, 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. 



1681. 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 insensibly stealing upon it, and 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 



