70 DISEASES OF FIELD & GARDEN CROPS. [CH. 



commonly makes itself apparent in midsummer by the 

 impoverished appearance of the flower-spike. Sometimes 

 the disease occurs in spring, and then it often proves fatal 

 to the plants attacked. 



Straw blight is caused by the attack of a fungus on 

 and in the straw at a point close to or very near the 

 ground. The fungus growth seldom reaches so far up the 

 stem as the second or third joint, the attack being more 

 frequent below the first joint from the root, and close to the 

 ground. It is superficially recognised by brownish disease- 

 spots outside the straw, as illustrated at AA, Fig. 25, 

 enlarged 5 diameters ; but if the straw is carefully cut 

 longitudinally with a sharp knife, it will be seen that the 

 disease is by no means superficial. The disease-spot goes 

 through the solid wall of the hollow stem, and in typical 

 examples the hollow part will be more or less filled with 

 loose flocculent material, as illustrated at BB. This floccu- 

 lence is really the mycelium or spawn of a fungus. Hav- 

 ing now obtained a clue to the nature of the so-called 

 blight, an excessively thin and transparent atom must be 

 sliced off from the exposed surface of one of the brown 

 disease-spots with a lancet, and this slice highly magnified. 

 If we enlarge this atom 200 diameters, and examine at it 

 as a transparent object under the microscope, we shall prob- 

 ably see it as illustrated at Fig. 26. The base of the 

 illustration shows the cells of the solid part of the straw 

 in transverse section, whilst the main part of the illustra- 

 tion shows the stem in longitudinal section : the bottom, in 

 fact, represents the base of the minute transparent atom 

 sliced off. We now see the spawn threads distinctly ; they 

 are transparent or nearly so, and so fine and attenuated 

 that (as may be seen by the thinner lines of the illustration) 

 they are less in thickness than the walls of the microscopic 

 cells, of which the straw stem itself is built up. They will 

 be seen to branch, and apparently pierce the cell-walls 

 both vertically and horizontally, and in old examples to 

 almost fill the hollow of the stem. A close examination 



