Hillside. 135 



On our left hand now is a potato field, patchy in appear- 

 ance, some parts green and healthy, others with the 

 characteristic brown spots scattered here and there which 

 denote the hated disease. In some instances the whole leaf 

 has decayed, and occasionally the stem has also been at- 

 tacked. This is the dreaded "potato blight," caused by 

 an excessively minute fungus, called Peronospora. We will 

 examine one of these brown spots, for what we shall learn 

 from it will, in most respects, be similar to what we should 

 find in many other parasitical fungi, such as bunt, rust and 



FIG. 26. POTATO BLIGHT (Peronospora) MAGNIFIED. 



ergot. We see by the aid of a lens that a large number of 

 upright filaments pass through the skin of the leaf, bearing 

 on their apices little knobs which contain six or seven 

 simple cells or spores. In the soft cellular part of the leaf a 

 number of little fibres interlace ; these absorb the sap in the 

 cells for their own use, causing the cells themselves to decay 

 and to give rise to the brown blotches which are so con- 

 spicuously evident. It is from these interlacing fibres that, 

 the upright stems grow, piercing the outside cuticle of the 

 leaf, and on these the spores are developed. The spores in 

 turn fall and germinate. The little rootlets pierce the skin 

 of the leaf, and form another network, which gives rise in its 



