420 H. MARSHALL WARD. 



were entering the leaf. The drawing is combined from two 

 preparations to save space^ the zoospore which is putting forth 

 a tube through the stoma having been observed on a different 

 part from the other — in other words, instead of only one cell 

 intervening between the stoma and the lower of the two germi- 

 nating zoospores, there were very many. The zoospore close 

 to the stoma simply protrudes its tube into the orifice. The 

 one lower down has germinated on an epidermis-cell, and the 

 tip of the tube at once commenced to bore through the outer 

 wall ; once inside, the germinal tube swells up and has in this 

 case branched. The empty remains of the zoospore and part 

 of the tube are left outside. The chlorophyll-corpuscles are 

 shown in the guard-cells, but the nuclei are omitted. 



The germination of the conidia on the living leaf is often 

 very rapid, and may certainly take place in two hours after the 

 sowing was made. 1 could not satisfy myself that it is af- 

 fected by light, as is that on glass slides. It is accomplished 

 readily during the night, on leaves kept wet under bell-jars. 

 The results seem to be more satisfactory if rainwater is em- 

 ployed, in preference to well-water ; and the same is true of 

 experiments on glass slips. It is improbable that temperature 

 was the important factor in these differences, possibly the 

 oxygen present in the rainwater was of more significance. 



It occasionally happens that a zoospore germinates in an 

 angle of the venation of the leaf, and sends its germinal hypha 

 through the epidermal cells of the rib or " vein " lying at its 

 side ; in such cases an optical section of the tube and zoospore 

 can be obtained, but the best proof of the entry of the germi- 

 nal tube through the wall of the epidermis-cell is obtained as 

 follows. 



Fig. 6. — The preparation is part of a vertical longitudinal 

 section of a young internode of the potato plant, and shows 

 a stoma in longitudinal but not quite median section, and to 

 the right a germinating zoospore, the germinal hypha of which 

 has pierced the cuticle and cell-wall and is growing on inside 

 the epidermis-cell. The method adopted was to sow large 

 quantities of the conidia on one of the flat sides of the tetra- 



