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have been expected, before Mr. Hales 

 demonstrated it by experiment. 



" Now, we know that a much less 

 pressure than any of those above men- 

 tioned would be capable of bursting the 

 delicate membranes of any of their ex- 

 terior descending sap vessels, and it is 

 in such outer ducts that the injury first 

 occurs. When one exterior vessel is 

 ruptured, that next beneath it, having 

 the supporting pressure removed, is 

 enabled to follow the same course at 

 the same locality; and in proportion to 

 the length of the time that the sap con- 

 tinues in excess, is the depth to which 

 the mischief extends, and the quantity 

 of sap extra vasated. 



" If the extravasation proceeds from 

 this cause, there is but one course of 

 treatment to be pursued ; sever one of 

 the main roots to afford the tree im- 

 mediate relief, and reduce the staple of 

 the soil, by removing some of it, and 

 admixing less fertile earthy components, 

 as sand or chalk. This must be done 

 gradually, for the fibrous roots that are 

 suited for the collection of food from a 

 fertile soil are not at once adapted for 

 the introsusception of that from a less 

 abundant pasturage. Care must be 

 taken not to apply the above remedies 

 before it is clearly ascertained that the 

 cause is not an unnatural contraction of 

 the sap vessels, because, in such case, 

 the treatment might be injurious rather 

 than beneficial. I have always found it 

 arising from an excessive production of 

 sap, if the tree, when afflicted by extra- 

 vasation, produces at the same time 

 super-luxuriant shoots. 



" 3. Local contractions of the sap 

 vessels. — If the extravasation arises 

 from this cause, there is usually a swell- 

 ing of the bark immediately above the 

 place of discharge. 



"I had a cherry tree in my garden, 

 in Essex, of which the stock grew very 

 much less freely than the graft, conse- 

 quently, just above the place of union, 

 a swelling resembling a wen extended 

 round the whole girth of the tree, from 

 which swelling gum was continually 

 exuding. In the stem below it, I never 

 observed a single extravasation. In a 

 case such as this, the cultivator's only 

 resource is to reduce cautiously the 

 amount of branches, if the bleeding 

 threatens to be injuriously extensive, 

 otherwise it is of but little consequence, 

 acting like temporary discharges of 



blood from the human frame, as a re- 

 lief to the system. 



" 4. The extravasation of the sap from 

 a wound is usually the most exhausting, 

 and as the wound, whether contused or 

 not, is liable to be a lodgment for wa- 

 ter and other foreign bodies opposed to 

 the healing of the injured part, the dis- 

 charge is often protracted. This is 

 especially the case if the wound be 

 made in the spring, before the leaves 

 are developed, as in performing the 

 winter pruning of the vine later than is 

 proper. In such case, the vine always 

 is weakened, and in some instances it 

 has been destroyed. The quantity of 

 sap which may be made to flow from 

 some trees is astonishing, especially in 

 tropical climates. Thus, from a cocoa- 

 nut palm, from three to five pints of sap 

 will flow during every day for four or 

 five successive weeks. The best mode 

 of checking such exudations, is by plac- 

 ing a piece of sponge dipped in a solu- 

 tion of sulphate of iron upon the dis- 

 charging place, covering the sponge 

 with a piece of sheet-lead, and binding 

 it on firmly. The sulphate acts as a 

 styptic, promoting the contraction of 

 the mouths of the vessels; the sponge 

 encourages cicatrization, and the lead 

 excludes moisture. 



" 5. Heat attended by dryness of the 

 soil, as during the drought of summer, 

 is very liable to produce an unnatural 

 exudation. This is especially notice- 

 able upon the leaves of some plants, 

 and is popularly known as honey-dew. 

 It is somewhat analogous to that out- 

 burst of blood, which in such seasons is 

 apt to occur to man, and arises from 

 the increased action of the secretory 

 and circulatory system, to which it af- 

 fords relief There is this great and 

 essential difference, that, in the case of 

 the plants, the extravasation is upon the 

 surface of the leaves, and conse(juently 

 in proportion to the abundance of the 

 extruded sap are their respiration and 

 digestion impaired. 



" Azaleas sometimes, but rarely, have 

 the impubcscence on their leaves, espe- 

 cially on their lower surface, beaded as 

 it were with a resinous exudation. This 

 can scarcely be called a disease. It is 

 never found but upon plants that have 

 been kept in a temperature too high, 

 and in a soil too fertile. It is an effort 

 to relieve the surcharged vessels, and 

 occurs in various forms in other plants. 



