DISEASES OF VEGETABLES. 



131 



produces similar effects. Du Hamel planted 

 some elms in a soil tliat was particularly well 

 manured, and accordingly they pushed with 

 great vigour for some time ; but at tlie end of 

 rive or six years they all died suddenly. The 

 bark was found to be detached from the wood, 

 and the cavity filled up with a reddish coloured 

 water. 



Some trees, but particularly tlie oak and birch, 

 are liable to a great loss of sap, either bursting 

 out spontaneously, owing to a superabundance 

 of juices, or issuing from accidental wounds. 

 Sometimes it is injurious to the health of the 

 plant ; while in other cases it has no such effect. 

 The tlieory of the ascent of the sap, as proposed 

 by Dutrochet, and already detailed in these 

 pages, may perhaps account for diseases of this 

 nature, by supposing an excess of the action of 

 endosmose over that of exosmose. 



There is a spontaneous extravasation of the 

 sap of the vine, known by the name of the tears 

 of the vine, which is not injurious, as it often 

 liappens that the root imbibes sap which the 

 leaves are not yet prepared to throw off, because 

 not yet sufficiently expanded, owing to an in- 

 clement season; tlie sap wlnich is first earned up 

 being propelled by that which follows, ultimately 

 forces its way through all obstructions, and ex- 

 udes from the bud. But tliis is observed only 

 in cold climates ; for in hot climates, where the 

 development of the leaves is not obstructed by 

 cold, they are ready to elaborate the sap as soon 

 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 chen-y, 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. But when 

 the sap ascends more copiously than it can be 

 carried off, it sometimes occasions a fissure of 

 the solid parts, inducing disease or deformity, by 

 encouraging the extravasation and corruption of 

 the ascending or descending juices. Sometimes 

 the fissure is occasioned by means of frost, form- 

 ing 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. Some- 

 times a layer is partially affected, and that is gene- 

 rally 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 albur- 

 num is split into clefts or chinks, by the expansive 

 force of the freezing sap. But a cleft thus oc- 

 casioned often degenerates into a chilblain, that 

 discharges a blackish and acrid fluid, to the great 

 detriment of the plant, particularly if the sore 

 is so situated that rain and snow will lodge in it, 

 and become putrid. The same injury may be 

 occasioned by the bite or puncture of insects, 



while the shoot is yet tender, and as no vegetabla 

 ulcer heals up of its own accord, the sooner a 

 remedy is applied to it the better, as it will, if 

 left to itself, ultimately corrode and destroy the 

 whole plant, bark, wood, and pith. The only 

 remedy is the excision of the part affected, and 

 the application of a coat of gxafting wax. 



Gangrene. There are two varieties of this 

 disease. The one arising from an excessive de- 

 gree of temperature; the other from the extreme 

 of cold. A very low temperature shrivels and 

 destroys the vitality of green leaves and shoots, 

 converting them from the natural green to a 

 black or brown. The inner bark also becomes 

 affected from the same cause, and thus the de- 

 struction of the whole plant follows. The effects 

 of excessive heat are nearly similar, as may be 

 witnessed in tropical climates, and in our very 

 hot summers; and even vmder ordinary heat, 

 when the roots of trees or vegetables are unduly 

 exposed to the sun. 



Sometimes gangrene is caused 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 oc- 

 casioned by parasitical plants, as in the case of 

 the bulbs of saffron, to which a species of li/co- 

 perdon often attaches itself, and totally corrupts. 

 The harmattan winds of the coast of Africa 

 kill many plants, by inducing a kind of gangrene 

 that withers and blackens the leaves, and finally 

 destroys the whole plant. 



Plants are sometimes affected with a gangrene, 

 by which a part becomes first soft and moist, 

 and then dissolves into foul ichor. This is con- 

 fined 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 origin- 

 ate in contusion, and may be caught by infection. 



Menonville, in his work on the culture of the 

 nopal, as the food of the cochineal insect, gives 

 several interesting notices of this disease. This 

 writer travelled many years ago through the 

 Spanish settlements in South America, chiefly 

 noted for the cultivation of the cochineal insect, 

 on purpose to transport it clandestinely to some 

 of the French islands. Such were the supine- 

 ness and ignorance of the Spaniards, that he 

 succeeded in conveying not only the living in- 

 sects, but the bulky plant necessary for their 

 sustenance, notwithstanding severe edicts to the 

 contrary. He had attended previously to the 

 management of the nopal, and made his remarks 

 on the diseases to which it is liable. Of these, 

 the gangrene is extremely frequent in the true 

 nopal of Mexico, beginning by a black spot, 

 which spreads till the whole leaf or branch rots 

 off, or the shrub dies. But the same kind of 

 plant is often affected with a much more serious 

 disease, called by Thieiy "la dissolution." This 



