ASCOMYCETES 251 



the knotted side. While the smaller twigs are usually affected, 

 the knot may also be found upon branches nearly two inches in 

 diameter. 



The fungus. The mycelium of the fungus is found during the 

 early stages occupying most of the cambium and the bast areas, 

 as well as extending throughout the cortex. If the whole cambium 

 ring becomes affected the girdling causes the death of the limb be- 

 yond. In general, however, the growth of the twig continues, since 

 the fungus is confined to one portion of the cambium, growing 

 from this layer towards the periphery. The knot itself is made 

 up of a mass of tissue, comprising, on the one hand, dense areas 

 of the fungus and, on the other, various cells or tissue elements of 

 'the host. Bast fibers, parenchyma cells, and even vessels may be 

 found in this heterogeneous mass in which all of the associations 

 of cells normally present have disappeared. This abnormal con- 

 dition is apparently brought about by the breaking up of the 

 cambium and a resulting development of all the various cell forms 

 to which it may give rise in the diverse isolated areas. The dis- 

 tribution of the fungous hyphae and the miaute anatomy of the 

 knot varies upon different hosts. 



During the development of the knot in the spring, small, 

 greenish areas may be noticed upon the surface, and later the 

 mycelium breaks through the bark from all directions and forms 

 upon the surface a very dense layer of closely adherent or pseudo- 

 parenchymatous cells. This stromatic fungous layer gives rise to 

 conidiophores, which are flexuous and septate (Fig. 108, c). Each 

 conidiophore produces a spore at the tip, and by further growth 

 scars, geniculations, or short branches may result. The conidio- 

 phores are produced in such quantity that the surface has a vel- 

 vety appearance ; they measure from 40-60 x 4-5 p. The conidia 

 are simple, and light brown in color. The period of conidial 

 production usually extends from late spring until midsummer. 

 Gradually, as the season advances, the velvety surface disappears, 

 disclosing a deep black stroma which has been gradually differ- 

 entiated. From an early period there can be observed with a 

 hand lens certain papillae which locate the forming perithecia 

 in this stromatic area. The later conidiophores are therefore 

 still evident on the surface when developing perithecia are easily 



