262 



SCIENCE OF AGRICULTURE. Tart It. 



h.,p wbicti i- Brsl carried up, being propelled by that which follows, ultimately forces its way through all 



ructions and exudes from the bud. Hut this is observed only in cold climates; tor in hot climates, 



where the developemeut ol the leaves Is not obstructed li> cold, they arc ready to elaborate the sap as 



toon a* it reaches them There is also ■ spontaneous extravasation of proper juice m some trees, which 



not seem in general to be injurious to the Individual Thus the gum winch exudes from cherry, 



plum, peach, and all .1 trees Is seldom detrimental to their health, except when it insinuates itself into 



the other vessels of the plant and occasions obstructions. , 



1701 lint the exudation «( gum i- sometimes a disease, and one for which there is seldom any remedy, 

 it is renerallj the consequence of an unsuitable soil, situation, or climate. Cold raw summers will pro. 



dUCC it in thep.Mch.apiie.it. and more tender sorts of plum and cherry ; or crafting these fruits on diseased 

 Stocks (lilting out the part and applying a covering of loam, or tar and charcoal, to exclude the air, are 

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

 a suitable soil and situation. ,..•■,, 



I7n ! The extravasation and corruption of the ascending or descending juices, have been known to occa- 

 sion a fissure ol the solid parts. Sometimes the fissure is occasioned by mean- ol frost, and forms what is 

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



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

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

 the alburnum Is split into chits or chinks, by means of the expansion of the frozen sap. 



1703 Chilblain* Hut clefts thus occasioned often degenerate into chilblains which discharge a blackish 

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

 Will readil] 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, hark, wood, and pith. The only palliative is the excision of the part affected, and the application 

 Ol a coat of grafting wax. [H'tlldenow, p. 354.) 



1704. 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 

 Ts 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 clear 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 Lycoperdon 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 rots off, or the plant dies. But plants are sometimes affected with a 

 gangrene bv 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"^ 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 yellow, 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 oft' 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 rentiers 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 to 

 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 Secale ccreale ; 

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



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

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

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

 disease is called nijffoeation* 



170S Sometimes it is occasioned by the immoderate growth of lichens upon the bark, covering the whole 

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

 oil' 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 freewill soon begin to languish, and will finally become 

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



171 1, Jiut a similar effect is also occasionally produced hy 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 Coccus Aesperidum and ./ cams t. In ius, 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 bv the hand or other mechanical means, or destroyed by excess of some ol He- 

 el. -incuts of (heir nutrition, as heat, cold, or moisture, where such e\i is, ,U<r> not prove injurious to the 

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



