369 SCIENCE OF AGRICULTURE. Part II. 



sap which k first carried up, being propelled by that which follows, ultimately forces Its way through all 

 obstructions, and exudes from the bud. But this is observed only in cold climates ; for in hot climates, 

 where the developemeut of the leaves is not obstructed by cold, they are ready to elaborate the sap as 

 ioon as it reaches them. There is also a spontaneous extravasation of proper juice in some trees, which 

 does not seem in general to be injurious to the individual. Thus the gum which exudes from cherry, 

 plum, peach, and almond trees is seldom detrimental to their health, except when it insinuates itself into 

 the other vessels of the plant and occasions obstructions. 



1701. But the exudation of gum is sometimes a disease, and one for which there is seldom any remedy. 

 It is generally the consequence of an unsuitable soil, situation, or climate. Cold raw summers will pro- 

 duce it in the peach, apricot, and more tender sorts of plum and cherry ; or grafting these fruits on diseased 

 stocks. Cutting out the part and applying a covering of loam, or tar and charcoal, to exclude the air, are 

 palliatives ; but the only effectual method, where it can be practised, is to take up the tree and place it in 

 a suitable soil and situation. 



1702. The extravasation and corruption of the ascending or descending juices, have been known to occa- 

 sion a fissure of the solid parts. Sometimes the fissure is occasioned by means of frost, and forms what is 

 called a double alburnum ; that is, first a layer that has been injured by the frost, and then a layer that 

 passes into wood. Sometimes a layer is partially affected, and that is generally owing to a sudden and 

 partial thaw on the south side of the trunk, which may be followed again by a sudden frost. In this case 

 the alburnum is split into clefts or chinks, by means of the expansion of the frozen sap. 



1703. Chilblains. But clefts thus occasioned often degenerate into chilblains which discharge a blackish 

 and acrid fluid, to the great detriment of the plant, particularly if the sores are so situated that rain or snow 

 will readily lodge in them and become putrid. The same injury may be occasioned by the bite or punc- 

 ture of insects while the shoot is yet tender; and as no vegetable ulcer heals up of its own accord, the 

 sooner a cure is attempted the better, as it will, if left to itself, ultimately corrode and destroy the whole 

 plant, bark, wood, and pith. The only palliative is the excision of the part affected, and the application 

 of a coat of grafting wax. ( WiUdenow, p, 351) 



1 704. Gangrene. Of this disorder there are two varieties, the dry and the wet. The 

 former is occasioned by means of excessive heat or excessive cold. If by means of cold, 

 it attacks the leaves of young shoots, and causes them to shrink up, converting them from 

 green to black ; as also the inner bark, which it blackens in the same manner, so that it 

 is impossible to save the plant except by cutting it to the ground. If by means of heat, 

 the effects are nearly similar, as may oftentimes be seen in gardens, or even in forests, 

 where the foresters are allowed to ckar away the moss and withered leaves from the 

 roots. Sometimes the disease is occasioned by the too rapid growth of a particular 

 branch, depriving the one that is next it of its due nourishment, and hence inducing its 

 decay. Sometimes it is occasioned by means of parasitical plants, as in the case of the 

 bulbs of the saffron, which a species of Lycop^rdon often attaches itself to and totally 

 corrupts. 



1705. Dry gangrene. The harmattan winds of the coast of Africa kill many plants, by means of 

 inducing a sort of gangrene which withers and blackens the leaves, and finally destroys the whole plant. 

 The nopal of Mexico is also subject to a sort of gangrene which begins with a black spot, and extends 

 till the whole leaf or branch rota off, or the plant dies. But plants are sometimes affected with a 

 gangrene by which a part becomes first soft and moist, and then dissolves into foul ichor. This is confined 

 chiefly to the leaves, flowers, and fruit. Sometimes it attacks the roots also, but rarely the stem. It 

 seems to be owing, in many cases, to too wet or too rich a soil ; but it may originate in contusion, and may 

 be caught by infection. But the nopal is subject also to a disease called by Thierry la dissolution, con- 

 sidered by Sir J. E. Smith, as distinct from gangrene, and which appears to be Willdenow's dry gangrene. 

 A joint of the nopal, or a whole branch, and sometimes an entire plant, changes in the space of a single 

 hour from a state of apparent health to a state of putrefaction or dissolution. Now its surface is verdant 

 and shining, and in an instant it changes to a yellov/, and its brilliancy is gone. If the substance is cut 

 into, the parts are found to have lost all cohesion, and are quite rotten. The attempt at a cure is by speedy 

 amputation below the diseased part. Sometimes the vital principle, collecting and exerting all its energies, 

 makes a stand as it were against the encroaching disease, and throws off the infected part. {Smith's In- 

 troduction, p. 276., edit. 6.) 



1706. Etiolation. Plants are sometimes affected by a disease which entirely destroys 

 their verdure, and renders them pale and sickly. This is called etiolation, and may arise 

 merely from want of the agency of light, by which the extrication of oxygen is effected, 

 and the leaf rendered green. Hence it is that plants placed in dark rooms, or between 

 great masses of stone, or in the clefts of rocks, or under the shade of other trees, look 

 always peculiarly pale. But if they are removed from such situations, and exposed ia 

 the action of light, they will again recover their green colour. Etiolation may also 

 ensue from the depredations of insects nestling in the radicle, and consuming the food of 

 the plant, 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 5ec^e cereale ; 

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



1707. Stiffvcation. Sometimes it happens that the pores of the epidermis are closed 

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

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

 disease is called suffocation. 



1708. Sometimes it is occasioned bp 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 perform 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. 



1709. 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 

 in the case of C6ccus Aesp^ridum and /f 'carus telarius, insects which 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, cold, or moisture, where such excess does not prove injurious to the 

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



