68 



small variations I mean comparatively small ; it will be seen by the 

 measurements given on this and succeeding pages what interpretation 

 must be put upon this expression, for it can be there seen what is the 

 actual variation among spores derived from the same mycelium. 



The earlier stages of this disease, as it occurs on the leaves of the 

 orange, are characterised by irregular roundish or elongated yellow 

 patches of considerable extent. These patches are from five to twenty- 

 five millimetres or more across. A striking feature of the spots is the 

 early appearance of reddish blotches on the yellow background. These 

 reddish blotches make their first appearance on the primary yellow of 

 the discolouration in the form of small reddish dots, which eventually 

 run together in the middle of the diseased spot, though they may 

 continue to appear as isolated dots near the margins. In consequence 

 of this a diseased area in its secondary stage has the following 

 appearance : A reddish central part bordered by small reddish spots 

 arranged on a yellow background. 



In the final stages of the attack the reddish central patch of the 

 secondary stage becomes ash-coloured, and bears the Alternaria, the 

 fructification of which assumes the form of minute tufts of dark green, 

 or almost black, aerial mycelium, consisting of unbranched hyphae, 

 bearing spores in chains. 



Around the central ash-coloured spore-bearing area is a brown 

 margin which pales through yellow to the normal green of the plant. 

 None of these growths cause any material thickening or swelling 

 of the leaf, which throughout the progress of the disease remains 

 normal in thickness. 



The adjacent measurements 

 show the range of size of the 

 Alternaria spores. The long- 

 est are 15 to 20 celled, while 

 the smallest are 2 to 4 or 5 

 celled. The adjoining wood- 

 cut shows some of the spores 

 from which these measure- 

 ments were made. The spores are invari- 

 ably smooth-walled. It may therefore be 

 said that the full-grown spores are 15-20 

 celled, and measure 12-18 x 60-70 /x. 



Small one- and two-celled spinous or 

 warty spores of the form referred to 

 Cladosporium, were occasionally seen 

 among the typical Alternaria spores, but 

 it was impossible to more than conjecture 

 the part played by them. Only a few were seen, not more than three 

 or four among many hundreds of the Alternaria spores. 



When pieces of infested orange leaf were placed in water-culture, 

 the mycelium of the Alternaria grew from the leaf tissue out into the 

 water and air, and these new growths produced spores after the 

 manner of Alternaria, i.e., in chains, and the spores produced under 

 these conditions did not differ very materially from those produced in 



70 

 62 

 40 

 25 

 22 

 18 

 20 

 25 



12 



18, 



15, 



12, 



10, 



10, 



12, 



10, 



Fig. 79. Spores of an Altemaria 

 found growing on the leaves of 

 the orange. 



