STEVENS: DISEASES OF GRASSES. I29 



such as those represented on page 124 are plentiful in the cells of the 

 sheath adjacent to the abnormal growth, within the new growth the 

 filaments are much smaller, and are discernible with difficulty except 

 where the branches are massed together for the production of spores. 

 The application of a solution of iodine to sections brings out promi- 

 nently an abundance of starch grains in the cells of the sheath. These 

 become very scarce, however, in the cells just beneath the new growth, 

 but are found massed together again in the cells immediately sur- 

 rounding the spore masses. Aside from the distortion of the cells, 

 the tissues of the corn in the vicinity of the fungus do not seem to 

 have suffered. There is no discoloration, no evident lack of healthy 

 nutrition. However, the nutriment used up in the production of 

 abnormal tissues, and in the formation of the large spore- masses of 

 the fungus, might have gone toward the upbuilding of the essential 

 parts of the plant. When the smut occurs in the pistillate flowers it 

 is the young ovaries that become the seat of the abnormal growths for 

 the formation of the spore-masses, and here the injury becomes more 

 evident, because done, to the very consummation of the plant's exist- 

 ence. 



Another class of injuries to the tissues of -the host plant is shown 

 by leaves of Panicum sangjiinale, parasitized by Piricularia grisea. 

 Fig. 2, plate XIII, illustrates this. The affected portion of the leaf 

 has shrunk to about one-third its natural diameter, and the collapsed 

 tissues are blackened a,s though by fire. As the fungus extends its 

 growth in a widening circle, the parenchyma cells, which radiate from 

 the smaller fibro-vascular bundles, melt down and lose their identity 

 in a darkened mass. Often the cuticle is blackened and shrunken, 

 and even the lignified portions of the bundles do not escape the gen- 

 eral discoloration. In this instance the spores are borne on pedicels 

 which grow out through the stomata, and the mechanical injury 

 caused by the parasite is slight. The chief injury is done to the 

 chlorophyll-bearing parenchyma. It is the elaborating, rather than 

 the conducting tissues that suffer, for frequently where a parasitized 

 spot occupies over two-thirds the breadth of the leaf the portion of 

 the leaf above the affected part retains its turgidity and apparent 

 healthfulness. How much the fungus draws upon the general circula- 

 tion of the host is not evident. It is clear, however, that it disin- 

 tegrates and feeds upon tissues already formed. 



The obliteration of the parenchyma of the leaf is sometimes caused 

 by the formation of oospores in large numbers. This is shown by 

 No. 64, in fascicle 11, of Seymour and Earle's Economic Fungi. The 

 example is a leaf of Setaria viridis parasitized by Feronospora gratni- 

 nicola. The dried specimen crumbles easily to long shreds as though 



