INJURIES INDUCED BY PLANTS 41 



tissues till the following year, behaves in a physiologically 

 similar manner. The fatty oil is not unfrequently coloured, and 

 especially in the case of " rusts," whose oil is of a golden yellow 

 colour, a yellow hue is imparted to the tissues of the leaves and 

 cortex in which the mycelium grows. Usually drops of cell-sap, 

 or so-called vacuoles, also appear at an early stage in the proto- 

 plasm, and these, by forcing most of the protoplasm against the 

 walls, impart a frothy appearance to the contents. 



It is only when nitrogenous food-materials are present in 

 abundance that the contents of the hyphae are retained for a 

 long time. This occurs when mycelia vegetate in or amongst 

 the tissues of the cortex, bast, or leaves, which for the most 

 part consist of parenchymatous cells. On the other hand, 

 the contents disappear early when the mycelium vegetates in 

 tissues containing little nourishment, as is markedly the case in 

 the wood of trees. When the mycelium of a fungus spreads in the 

 interior of a tree, it finds abundance of nitrogenous food-materials 

 in the contents of the cells of the medullary rays and the wood- 

 parenchyma. It is thereby enabled to produce vigorous hyphae, 

 even when traversing the empty lumina of trachei'des, wood- 

 fibres, or vessels. When the hyphae have to pass through 

 regions of tissue containing no proteids, their apices are supplied 

 with protoplasm which is sent forward from behind at the 

 expense of the older parts of the hyphae. The latter are 

 therefore soon emptied, and become filled with air. Although 

 the empty mycelial hyphae persist for some time, they ultimately 

 disappear under the decomposing influence of the fungus itself. 

 The consequence is that one may frequently fail to find anything 

 of the fungus itself, although numerous punctures in the walls of 

 the cells show clearly that the fungus had formerly been present 

 in that part of the tissues. In proportion as the mycelium 

 develops in the wood, so does a dearth of proteids for the 

 production of new fungus-protoplasm set in, and this is 

 strikingly manifested in the diminished thickness of the new 

 hyphae. 



The walls of the hyphae, which consist of fungus-cellulose, 

 are at first very delicate, though in the course of time they 

 occasionally attain such a thickness that the lumen almost 

 entirely disappears. In this way it sometimes happens that a 



