13 



rally can be readily determined by comparing PL III, which is a photo- 

 graph of a naturally diseased and a healthy plant from greenhouses 

 at Garrett Park, Md., December 18, 1897, with PI. IV, which is a 

 photograph of a healthy and an artificially infected plant. The simi- 

 larity of the spots is, however, more strikingly shown in PI. I. Fig. 1 

 is a healthy leaf; fig. 2 is a diseased leaf from a plant naturally infected 

 with the disease, while leaves 3 and 1 were taken from the diseased 

 plant shown in PI. IV, which was artificially infected with the disease. 



Spores of the fungus Alternaria violw sown in water in a van Tieg- 

 hem cell and kept at a temperature of 65° to 80° F. germinate readily 

 in from one and a half to three hours. Fig. 9, PI. VI, is a camera 

 lucida drawing of a group of spores that were sown in distilled water 

 and placed in a van Tieghem cell at 10 a. m., January 15, 1898. The 

 dotted lines at right angles to several of the germ tubes mark the 

 amount of growth made by them between the time of sowing and the 

 time noted; all subsequent growth and the production of all germ 

 tubes not marked with a dotted line occurred between 11.55 a. m. and 

 2.10 p. m. the same day. Figs. 1 to 6 show a camera lucida drawing of 

 a group of Alternaria spores in distilled water just previous to being- 

 placed in a van Tieghem cell, and figs. 7 and 8, two spores in the same 

 sowing, nineteen hours later. Figs. 11, 13, and 11: show spores that 

 were sown in distilled water in a van Tieghem cell at 10.20 a. m., 

 August 19, 1898, at which time they showed no signs of germinating. 

 Four hours later, however, at 2.20 p. m., the time at which the draw- 

 ings were made, the number of germ tubes had developed as indicated. 

 Fig. 12 is a second drawing of fig. 11, made twenty minutes later. 

 Fig. 10 is a camera lucida drawing of two spores from four to six hours 

 after being placed in distilled water. Figs. 15, 16, and 17 show the 

 chain formation of spores and their attachment to the mycelium. 

 This drawing was made from a pure plate culture of the fungus. 



Numerous greenhouse and laboratory experiments under strict con- 

 trol conditions have confirmed these results, and show that spot disease 

 of the violet is due directly to the attack of the parasitic fungus 

 Altermiria violce, and not to any of the other causes suggested. Indi- 

 rectly, however, other conditions may have their efi'ect. Any one or 

 a combination of all of the conditions included in the various theories 

 advanced may cause the plants to become susceptible to the attacks of 

 the fungus. 



CONDITIONS FAVORING THE DEVELOPMENT AND SPREAD OF THE DISEASE. 



The conditions favoring the development and spread of the fungus 

 may be considered under two heads, viz, natural conditions and arti- 

 ficial conditions. 



Among natural conditions those of the damp, warm, cloudy weather 

 of the summer season are the most diflicult to modify or control. 



