THE PEAR. 647 



difficulty ; it chokes up the sap-vessels, freezes and thaws again, loses its 

 vitality, and becomes dark and discolored, and in some cases so poison- 

 ous as to destroy the leaves of other plants when applied to them. 

 Here, along the inner bark, it lodges, and remains in a thick, sticky 

 state all winter. If it happens to flow down till it meets with any ob- 

 struction, and remains in any considerable quantity, it freezes again 

 beneath the bark, ruptures and destroys the sap-vessels, and the bark 

 and some of the wood beneath it shrivels and dies. 



In the ensuing spring the upward current of sap rises through its 

 ordinary channel, the outer wood or alburnum, the leaves expand, and, 

 for some time, nearly all the upward current being taken up to form 

 leaves and new shoots, the tree appears flourishing. Toward the begin- 

 ning of summer, however, the leaves commence sending the downward 

 current of sap to increase the woody matter of the stem. This current, 

 it will be remembered, has to pass downward through the inner bark or 

 liber, along which still remain portions of the poisoned sap, arrested 

 in its course the previous autumn. This poison is diluted, and taken up 

 by the new downward current, distributed toward the pith, and along 

 the new layers of alburnum, thus tainting all the neighboring parts. 

 Should any of the adjacent sap-vessels have been ruptured by frost, so 

 that the poison thus becomes mixed with the still ascending current of 

 sap, the branch above it immediately turns black and dies, precisely as 

 if poison were introduced under the bark. And very frequently it is 

 accompanied with precisely the odor of decaying frost-bitten vegetation.* 



The foregoing is the worst form of the disease, and it takes place 

 when the poisoned sap, stagnated under the bark in spots, remains 

 through the winter in a thick, semi-fluid state, so as to be capable of 

 being taken up in the descending current of the next summer. When, 

 on the other hand, it collects in sufficient quantity to freeze again, burst 

 the sap-vessels, and afterwards dry out by the influence of the sun and 

 wind, it leaves the patches of dead bark which we have already described. 

 As part of the woody channels which convey the ascending sap probably 

 remain entire and uninjured, the tree or branch will perhaps continue to 

 grow the whole season and bear fruit, as if nothing had happened to it, 

 drying down to the shrivelled spots of bark the next spring. The effect 

 in this case is precisely that of girdling only, and the branch or tree 

 will die after a time, but not suddenly. 



From what we have said, it is easy to infer that it would not be diffi- 

 cult, on the occurrence of such an autumn, when sudden congelation 

 takes place in unripened wood, to predict a blight season for the following 



* "We do not know that this form of blight is common in Europe, but the 

 following extract from the celebrated work of Duhamel on fruit-trees, published 

 in 1768, would seem to indicate something very similar a long time ago. 



"The sap corrupted by putrid water, or the excess of manure, bursts the 

 cellular membranes in some places, extends itself between the wood and the 

 bark, which it separates, and carries its poisonous acrid influence to all the 

 neighboring parts, like a gangrene. When it attacks the small branches, they 

 should be cut off ; if it appears in the large branches or body of the tree, all the 

 cankered parts must be cut out down to the sound wood, and the wound covered 

 with composition. If the evil be produced by manure or stagnant water (and it 

 may be produced by other causes), the old earth must be removed from the 

 roots, and fresh soil put in its place, and means taken to draw off the water 

 from the roots. But if the disease has made much progress on the trunk, the 

 tree is lost." Traite des Arbres Fruitiers, vol. 11, p. 100. 



