176 REV. R. ABBAY ON HEAirLEIA YASTATRIX. 



in spots or patches, easily detected when the leaf is held up to the 

 light. These quickly assume a faint yellow colour, and presently 

 become covered with bright yellow dust, which soon turns to a 

 rich orange. These are the ripening spores, or rather sporanges, 

 of the fungus aggregated in little clusters, just visible to the un- 

 assisted eye, as shown in Plate XIII. fig. 1, which is a drawing 

 taken from nature of a portion of a diseased leaf. One or two 

 points of interest are suggested by a superficial examination of 

 the character of the disease-spots. In the first place, they are 

 nearly circular in form, the central portion being the oldest and 

 losing its bright orange colour earliest, the black spot in the 

 centre showing w r here an aspergillus has fixed itself on the 

 more mature sporanges. The circular form and the regularity 

 of the colour of these spots at equal distances from the centre 

 suggests that the fungus has developed pretty equally on all sides 

 from some central point. Again, it may be noticed that the mid- 

 rib and nerves of the leaf form barriers, sometimes, but not 

 always, beyond which the fungus-spot does not pass. This sug- 

 gests that the infection must come from without, and not from 

 the juices within the leaf itself; for it is improbable, if the latter 

 were the case, that the nerves of the leaf could form barriers 

 beyond which the disease-spot could not spread. It seems natu- 

 ral, therefore, to suppose, and the hypothesis is borne out by mi- 

 croscopic observations, that each disease- spot is the result ot a 

 germinating body which has fixed itself at a point which after- 

 wards is the centre of the spot. 



Microscopic Examination of Dry Specimens*. — If, in order to 

 examine the character of the orange-red sporanges by means ot 

 transmitted light, the under cuticle of the leaf be torn oif in the 

 neighbourhood of a " disease-spot," the fungus will be found to 

 present the appearance shown in PI. XIII. fig. 2, each cluster 

 (A) consisting of a number of orange-red sporanges and occupying 

 a stomate of the leaf. Not infrequently every stomate is thus 

 occupied, and the cuticle appears to be covered with an unbroken 

 layer of sporanges. If now the specimen be turned upside down, 

 so that we look at the inner surface of the cuticle, several indis- 

 tinct and dark bodies are seen (fig. 3) immediately above the 

 clusters just mentioned. From these bodies a branching myce- 

 lium, more or less charged with reddish-brown granular matter, 



Nearly all the observations have been made by means of a T ymch immer- 

 sion-objective. 



