Black-Knot of Plums and Cherries. 



64: 



the places in which one year's growth stopped and that of the fol- 

 lowing year began, Fig. 3, 2. A careful examintion of the forks 

 of the small limbs shows that the union of the two parts is not so 

 close as at first appears. The cellular structure of these parts, as 

 revealed by the microscope, is seen to be such that the spore of a 

 fungus would apparently have less difl&culty in gaining an entrance 

 here than in almost any other place. Figure i , A, represents a sec- 

 tion made at the union of two twigs. The different parts of the limb 

 are here shown so that their relative positions may be understood. 

 The outer la5^er ck is composed of cork cells. These are penetrated 

 with difficulty by the threads from a germinating spore, and it may 

 be that entrance is entirely prevented by them. The layer lying 

 under the one composed of cork cells is the cortex, c 0. This 

 layer as a rule contains no cells which are lignified, or woody, but 

 is composed of cells which are easily penetrated by the mycelium 

 of many fungi. Underneath the cortex is a la^^er known as the 



I. — Crotch of branches where knots often form. 

 phloem. The phloem comprises the hard and the soft bast cells, 

 only the former being woody. The soft bast cells can also be pene- 

 trated by the threads of the mycelium and offer no obstacle to the 

 passage of such parasites. In this manner a fungus can penetrate 

 without much difficulty to the cambium layer, c a, represented 

 only by a dark line, when once the layer of cork cells has been 

 passed ; for the woody or lignified cells are arranged in isolated 

 bundles so that they do not interfere with the inward growth of the 

 threads of the mycelium, or vegetative portion of the fungus. 

 Diagram B of Fig. i represents only the layer of cork, more high- 



