14 



ing a new layer of wood unci bark, and, on the other, irritated by 

 the presence of the fungus, it lias produced a mass, the knot, in 

 Avliich all distinction between wood and bark has been lost. The 

 condition of the interior of the knot, in its later stages, is modified 

 very much by the depredations of insects, as well as by the drying 

 and crumbling of the tissue itself. On the wild cherries, the outer 

 part is generally a mere shell and tlie internal part is nearly empty. 

 On the i)lum, the interior is more apt to ho honeycombed ; and 

 we do not unfrequcntly have a very hard layer next the wood, com- 

 posed of thick-walled, dotted cells. The knot on cultivated cherries 

 is intermediate between that on wild cherries and that on plums. 

 The condition of the interior of the knot has an important l)earing 

 on the propogation of the disease. Where it is hollow and the outer 

 part brittle, as in the choke cherry, it is easy to see that the spore- 

 bearing portion Avill reatlily be broken up, and the spores blown 

 about without difficulty. AVhen it is more solid, as in the plum, 

 the spores will be disi)laced less easily ; and we can see how by the 

 action of the weevil or curculio in l)oring into the knot and mak- 

 ing the tissue more spongy, it is facilitating the dispersion of the 

 spores of the fungus when they shall ripen ; and on the other hand, 

 the knot tends to increase the uuml)er of curculios, by offering a 

 suitable place for the deposit of their eggs. As a rule the trees once 

 attacked by the knot grow more and more diseased, l)oth l)y the ex- 

 tension downward and upward on the stem of the old knots, and by 

 the production of new ones from the germination and growth of 

 the old knots on other branches which have Ijeen proviously free 

 from them. ]\rore and more of the smaller l)ranches are killed l)y 

 the girdling effect of the knots and the nutrition of the larger 

 is evidently so decidedly impaired that the trees bear no fruit and 

 ultimately die. In a few cases the branches seem able to recover 

 from the scars Avhich indicate the previous seat of the disease. 



This fungus has its proper home on our wild species of cherry 

 and plum and from thence attacks our cultivated varieties. The 

 fungus is the same on either cherry or plum, from direct experi- 

 ment Prof. Farlow has proven that spores on the choke cherry will 

 germinate and produce knots in healthy plum trees. The three kinds of 

 North American trees on Avhich tlie black knot naturally occurs are 

 the choke - cherry (^Prunus Virginiand) ; the bird-cherry {Prunus Pen- 

 xylvanica) ; the Avild plum {Prunus Americana). It appears that no 

 variety of the cultivated plum is not subject to the attack of the 

 black-knot, while some varietv of the cultivated cherry seem to resist 



