274 THE CULTIVATION OF THE 



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 beginning of summer, 

 however, the leaves commence sending the downward 

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

 This current has to pass down through the inner bark of 

 liber along which still remain portions of the poisoned 

 sap, arrested in its course the previous fall. 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 the poison thus becomes mixed with the still 

 ascending current of sap, the branch above it immedi- 

 ately turns black and dies, precisely as though poison 

 had been introduced under the bark. In a note in Down- 

 ing, he quotes Duhamel as saying : The sap corrupted 

 by putrid water or 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 influences to all the neighboring parts 

 like a gangrene. Now these descriptions are graphic 

 and true to nature, and the causes given are plausible as 

 far as they go, indeed, I believe are true, but they just 

 stop short of the real cause, as will be seen further on. 

 Pears which mature their wood early, as the Seckel and 

 Duchess, making short and firm joints, are the pears 



