THE NINETEENTH ANNUAL MEETING. 



91 



thought to be caused by iusects, 

 as the ]>lum curculio and others 

 frequently deposit their eggs in the 

 excrescences, soon after they form^ 

 and the larvae, later on, hollow out 

 the knots. 



" The fungus was first described 

 by Schweinitz, who called it Sjihd'ria 

 morhosa. Afterwards, however, it 

 was placed in the genus PJowr'ujldia. 



"Even were it not known to be a 

 fungus, it can be shown that it was 

 not the work of insects. Among 

 the proofs commonly given are, (1) 

 the galls caused by insects have no 

 resemblance to the black-knot; (2) 

 the insects found in the knots are of 

 various species, and are often found 

 on trees that do not show the knots ; 

 ( 8 ) knots are often found in which 

 there are no insects, and the insect 

 larv?e are never found until the 

 nots are half grown. 



''While this of itself proves that 

 the insects are not the cause of the 

 sw^ellings, it may be added that, on 

 the other hand, the microscope will 

 reveal in every black-knot the char- 

 acteristic workings of the fungus. 



" As the knots on the cherry have 



Fig. 1. Black Knot. Plou-rightia morhosa, 8acc. „i- .i,fi,, ,1i#'^i.Q-i-.f QT^»T^oQTQnr.o fv/^-iTi 

 1. Stem of plum tree with knot upon it, as it appears ^ Sllglltiy Cllttereut appeaiauce tlOm 



in the fall and winter those ou the plum, it has bccu claimed 



i. Perithecium with mycelium, a. n, between the , j i p n i ji t 



cells of the stem, and covered with filaments bearing that the lUUgUS that CaUSeS the dlS- 

 spores, b, at their extremities. Section made in May. „ ^ ,• ,^j j-i, ^ „„,„, ,•„ 1^^4-1^ ^„„^r, 



3. Filaments and spores, (.couidia) more highly ease IS UOt the Same Ul both CaSCS. 



magnified. The micToscope, liowever, sliows that 



*. Section through a cavity containing stylospores. , . -, ^. -, . i ■ , 



After Fariow. they are identical m their structure. 



" The fungus appears on one side of the branch, and as nature makes an 

 attempt to repair the injury to the tissues, the entire branch is often 

 greatly swollen. The knots are usually from two to four inches in length, 

 l)ut sometimes they reach a length of ten inches or more. They are first 

 noticed in the spring, before the buds start. They appear as slight swell- 

 ings, and, bursting the epidermis, they enlarge rapidly and form a green^ 

 pulpy mass, which soon solidifies and gradually turns brown. If a section 

 of a young knot be examined in the early spring, there will be found in 

 the cambium, or new layer of wood, twisted bundles of mycelial threads. 

 These feed on the juices designed for the nourishment of that portion of 

 the branch, and it will not form a new layer of wood, although the other 

 side of the branch, if there are no threads present, will make the usual 

 growth. 



" Soon after the knot has formed it will be found thickly dotted with 

 minute rounded protuberances. These are the peritlwcia, and a pocket 

 microscope will show them to be covered with fine hair-like appendages, at 



