THE PEAR. G47 



difficulty ; it chokes up the s;ip-vcsscls, iVeez(^s and thaws again, loses its 

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

 ous as to destroy the leaves of other plants when ap[)lied to thcMn. 

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

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

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

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

 and some of the wood beneath it shrivels and dies. 



In the ensuing spring the upward current of sap rii;es through it.-- 

 ordiuary channel, — the outer wood or alburnum, — the leaves expand, and. 

 for some time, nearly all the upward current being taken \ip to foiin 

 leaves and new shoots, the ti-ee 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 remend)ered, has to pass downward through the inner bark or 

 liher, 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 downwai'd current, distribiited toward the pith, and pJong 

 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 tlie still asceniling current of 

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

 if poison were introduced under the bark. And vei-y 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 placv-. 

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

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

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

 on the othei- 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 luu'e already described. 

 As pai-t of the wo'>dy channels which convey the ascending sajj probably 

 I'eniain entire and uninjui'ed, 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 etiect 

 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- 

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

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



* We do not know that this form of blight is comraon 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 

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

 Dark, which it separates, and carries its poisonous acrid influe:ice to all the 

 neighboring parts; like a gangrene. When it attacks the small 1)i-anchGs, 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 l)y 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 trunl?, the 

 tree is lost." — Traili des Arbres Fruiders, vol. 11, p. 100. 



