RUST OF ANTIRRHINUM. 49 



The third agency by which the fungus is disseminated is watering. Car- 

 nation growers in watering make an effort not to wet the fohage any 

 more than is necessary, usually employing a stiff piece of hose or pipe 

 which enables them to get water into the middle of the bench without 

 wetting the plants above. To this simple practice is due in part the 

 decline in importance of carnation rust. The writer selected twenty 

 snapdragon plants of the same variety, all showing uredinia in approxi- 

 mately equal numbers. Ten of these plants were watered only on the 

 soil, no water touching the foliage. The other ten were treated the 

 same, except that their foliage was kept wet. After three weeks the 

 plants with wetted foliage showed 200 per cent more uredinia than they 

 had at the beginning, while the plants whose foliage had been kept dry 

 showed no increase in the number of uredinia. Water is necessary for the 

 germination of the spores and for infection. A carelessly directed stream 

 from a hose loosens spores from pustules, carries them to other plants, 

 and provides them with the necessary moisture for their germination. 



Pathological Anatomy. 



The upper epidermis of the leaf and the palisade cells are only rarely 

 affected by this disease. Occasionally a few palisade cells are forced 

 apart by hj^jhse. The spong>^ parenchyma cells are principally affected. 

 The parench>Tna cells in the immediate vicinity of a sorus do not attain 

 their normal size. Strands of hyphse force them apart and sometimes 

 cause them to grow into abnormal forms. The chloroplasts fade slightly, 

 but the yellow appearance of the area surrounding a sorus is due mostly 

 to the presence of a stroma of mycelium. After the intercellular my- 

 celium has become well established it develops this firm stroma in contact 

 with the lower epidermis, and often includes scattered spongy parenchyma 

 cells (see Fig. 5, Plate 2). This growing stroma and the rising pedicels 

 of the urediniospores finally rupture the epidermis. The contents of cells 

 containing haustoria do not degenerate unless the whole leaf becomes 

 involved. Attacked cells do not swell, and any hypertrophy on the 

 leaf is due to the development of stromata. Leaf cells of snapdragon 

 which are normally pink lose this pigment when attacked by the fungus. 

 When the sori occur on the stem the epidermis is ruptured, the cortical 

 cells are forced apart, and in some cases the mycelium may be found 

 between the cells of the fibro- vascular bundles. Ordinarily, however, 

 cells as far in as the phloem are not attacked. The chloroplasts fade 

 even less in the stem than in the leaves. The cells of the cortex do not 

 attain their normal size. Epidermal cells appear unchanged, though 

 raised as a membrane above a sorus. jSIycelium in both leaf and stem 

 is local. 



