212 BOARD OF AGRICULTURE. 



ing threads, called the mycelium which penetrate throughout the 

 tissues of the potato plant, absorbing its nourishment from the cells 

 of which those tissues are composed, causing them to decompose, 

 and the whole substance to become a foul and fa'tid mass. 



To understand the position which the fungus occupies in a potato 

 leaf let us examine the normal structure of the leaf. If avc make a 

 vertical section through the leaf of a potato and examine it under a 

 moderately high power of the microscope, (See Fig. 1) we see that 

 the upper and lower surfaces are each covered with a single layer 

 of colorless cells, forming what is called the epidermis of the leaf, 

 and from the surface of the under side arise those hair-like bodies 

 which give to it the peculiar hairy ai)pcarance readily seen In' the 

 naked eye. Besides the hairs on the under side of the leaf there 

 are minute openings thi'ough the epidermis, called breathing pores 

 or stomats, which are guarded by peculiarly formed cells which open 

 or close the breathing pore according as the air is moist or dr3-, thus 

 regulating the evaporation from the leaf. The interior of the leaf, 

 between the epidermis of the upper and lower surfaces, is composed 

 of a mass of cells of a more or less globular form, according as thej' 

 are more or less crowded and compressed into other than the normal 

 globulur form. The first layers of these cells immediately beneath 

 the epidermis of the upper side of the leaf are more compactly placed 

 than those below, and form a denser tissue on the upper than on the 

 lower surface of the leaf, and they do not permit so free a circula- 

 tion of the air. These cells contain in their protoplasm a mass of 

 granular matter of a bright green color called cldorophyl., and here 

 is where the food of the plant is elaborated. There are also bundles 

 of woody tissue scattered hei-e and tliere through this part of the 

 leaf not shown in the figure, forming the veins and veinlets. 

 Through this portion of the leaf the fungus pushes its m^-celium in 

 long fine threads between the green cells of the leaf, and contrasts 

 in color with the green cells in being of a light brown color tliough 

 of a much smaller diameter than the cells tliemselves. As the 

 mA-celium rests against the side of a cell, it gives off, occasional!}', 

 little projections called haustoria, which piess upon the side of the 

 cell, or even penetrate into it, b}- means of which the mycelium 

 draws its nourishment from the cell-, which becomes more and more 

 exhausted, and at last is completely destroyed. 



After the fungus has developed to a certain extent branches are 

 given off which extend out into the air through the breathing pores 



