156 FUNGOUS DISEASES OF PLANTS 



The fungus. The mycelium is abundant in the intercellular 

 spaces, varying in diameter from 8 to 1 2 /u, but frequently it is 

 of less diameter in the more compact tissue of Vitis vinifera. In 

 the leaves the fungous hyphae may be found throughout the part 

 affected, except in the woody parts of the bundles of the veins and 

 in the stem. They occur also in all tissues except the xylem. The 

 haustoria are of the characteristic knob shape. The hyphae are 

 somewhat more densely assembled in the vicinity of the stomata, 

 and through the stomata there emerge several sporophores (Fig. 

 53), each becoming constricted in its passage through the open- 

 ing, but subsequently attaining practically normal or more than 

 normal diameter, therefore often showing a bulbous base. At 

 maturity they are irregularly branched, the central axis giving 

 rise to lateral offshoots and sometimes the axis itself may be 

 lost, due to the preponderance in growth of the branches. The 

 method of branching and sporangial production has been care- 

 fully worked out by Farlow, according to whom " the branches, 

 which are few in number, generally from four to eight, are 

 placed alternately on the upper third of the axis, being generally, 

 but not always, distichously arranged. Relatively to the main axis, 

 they are all short, the broadest expansion from side to side not 

 being usually greater than .12 mm. The branches are furnished 

 with branchlets of a second and third order" (Fig. 53, b). 



In this species germination of the zoosporangia takes place 

 in water in about one and one quarter hours. The process, as 

 summarized from Farlow's careful studies of this phenomenon, 

 is about as follows : Spores produced during the night and put 

 to germinate during the early morning on slides containing a 

 few drops of water show first at the end of an hour the be- 

 ginning of segmentation of the protoplasmic contents, each seg- 

 ment having a single nucleus. These round themselves off into 

 oval bodies, which are massed at the distal end of the sporan- 

 gium, and in time they escape by dissolving or rupturing the 

 cell wall. The zoospores pass out, generally one at a time, re- 

 main quiescent a moment in becoming free, and then swim off 

 rapidly, each as a mature zoospore, provided with two lateral cilia, 

 projecting usually anteriorly and posteriorly (Fig. 53, c). In gen- 

 eral, the zoospores are more or less ovate, but the form in the 



