28 



abundant septate and branched mycelium, or opalescent thread- 

 like roots which grow in the midst of the cells of the leaf, and 

 •eventually appear through the stomata as fertile stems ; these 

 patches are seldom seen on the upper side of the leaf, on account 

 of the scarcity of the stomata on that side, and because the 

 cuticle is too thick and too strong for the delicate threads 

 easily to penetrate. The mycelium quickly proceeds towards 

 the stem ; the stem then becomes spotted with dark brown 

 patches and very soon the tuber is reached and affected. In a 

 day or two, and sometimes in a few hours, the disease which has 

 attacked the leaf proceeds from within, outwards, to bear fruit 

 in conspicuous white tufts. Almost as soon as the white tufts 

 appear on the leaves, brown specks become visible in the tissue 

 of the tuber, the starch grains are affected and discolored by 

 them, the specks become darker and eventually flow together, 

 the cell construction becomes dissolved and the potato is 

 speedily converted into a dark brown semi-liquid mass. 



If a section of a diseased leaf is made and observed with 

 an eighth of an inch objective, the hyaline or opalescent 

 mycelium or root threads will then be observed creeping 

 amongst the enclosed cells of the leaf, and making their way 

 towards the stem of the plant, branching, increasing, and 

 bearing fruit as they extend. 



The fertile stem, i.e., that part of the plant which appears 

 externally through the stomata may also be observed growing 

 from the mycelium, and eventually through the stomata. This 

 fertile stem is very thin, transparent, and is gradually 

 •attenuated upwards, it is supplied with from one to five 

 branches, either simple or with short branchlets, not bifid, nor 

 trifid. There are singular nodose swellings on the branches. 

 The tips of the branches are furnished with acrospores or 

 sheaths, and these contain zoospores furnished with cilia 

 apparently for locomotion. 



The P. infestans appears to be propagated by at least two 

 different processes, each of them being productive in a high 

 degree ; these are, by acrospores on the fertile stems, and by 

 resting spores on the mycelium ; they have thus a slight but 

 curious illustration in the potato plant itself, i.e., the potato 

 apple, the fruit ; and the potato itself, the tuber. 



The acrospores just mentioned as growing on the tips of the 

 branches of the fertile stem are ellipsoid or ovoid in form, 

 papillate at the apex, and slightly pedicellate at the base, one 

 square line will contain 3,270 of them. These acrospores are 

 sometimes called privileged spores or swarm spores. 



It would appear that the acrospores are themselves capable 

 of germinating under certain conditions, but more frequently 

 important changes take place in the granulous matter with which 

 they are filled. The granulous matter appears to differentiate, 



