Minnesota Plant Diseases. 85 



Fungi are even known to excite the formation of leaf-green in 

 plant parts usually devoid of it, e. g.. in petals of flowers, as is 

 the case in the white rust on mustard plants. In this case it is 

 very probable that we see simply the stimulation to the devel- 

 opment of latent beginnings of the leaf-green bodies, just as the 

 stamens and pistils are sometimes formed in flowers usually de- 

 void of them. As leaf-green furnishes the machinery for starch- 

 making, one sees that the amount of starch formed in a fungus- 

 inhabited part may vary with the fungus. However, leaf-green 

 is not the only agency of starch production. There are other 

 agencies for the transformation of starch from other com- 

 pounds. Aside from the effect of the fungus, upon leaf-green, 

 fungi react directly upon the starch, producing certain chemical 

 substances which dissolve the starch. Some fungi use all of 

 the available starch as soon as it can be reached, while others 

 cause a great accumulation of starch temporarily and dissolve 

 it in the important stages of their life history, during and just 

 preceding the formation of spores. A great many fungi are 

 able to dissolve starch and among them may be mentioned cer- 

 tain rusts, black fungi, white rusts and many wood-destroying 

 fungi. 



When tissues of plants are examined under the microscope 

 a honeycomb-like structure of cells is seen. The walls of most 

 cells are whitish, soft and composed of a substance called cellu- 

 lose. The walls of the cells of woody tissue enclose in the 

 youngest stages the protoplasm, but soon lose the latter. The 

 "woody" character of wood tissues is imparted by the thickness, 

 size and form of their cell walls, and the chemical compounds 

 found in them. In young stages, the wall is whitish and not 

 particularly resistant nor hard, for it is a cellulose wall. Later 

 new substances are added, which collectively are known as lig- 

 nin. and the tissues then become woody. But woods differ 

 among themselves due to variation in the above-mentioned 

 characters. The cellulose membranes are sometimes pierced by 

 fungus threads in a mere mechanical fashion, just as one would 

 force a pin or needle through them. Wood membranes offer 

 considerable resistance to most fungi, but some of the latter 

 have solved the problem of penetration of these walls. Such 

 are the wood-destroying fungi already mentioned. The 



