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ACCIDENTS, DISEASES, PARASITES. 173 



If we take a portion of the white efflorescence from a 

 vine leaf attacked by mildew, and examine it under a micro- 

 scope we soon recognise that it is formed by fructiferous 

 filaments covered with spores. These filaments come out 

 ~of the stomata in bunches, and this explains why they are 

 found in great numbers on the under face and only along 

 the veins of the upper face. They are connected with the 

 vegetative part of the fungus, by the mycelium which is 

 ramified between the cells of the parenchyma, from which 

 it derives its food. In certain points swellings takes place, 

 which, after being fecundated, give rise to winter spores 

 or eggs. 



Mycelium: The vegetative part of the Plasmopara or 

 mycelium corresponds to the white underground part of a 

 large mushroom and lives inside the tissues of the leaf 

 (Fig. 150 a). It is a continuous tube without partitions. 

 The mycelium draws all the food necessary to the fungus 

 from the inside of the cells of the leaf by means of spherical 

 suckers, which pierce the cell membrane. At the beginning 

 of spring the mycelium, gorged with nutritive matters, 

 swells in places. Some of these swellings are spherical ; 

 others are smaller and irregular, and by their unions they 

 give birth to the reproducing body (pospore, ova, winter 

 spores). 



The mycelium strongly resists outside agents. Fre"chou 

 proved that under certain circumstances it could remain 

 during winter in the dead leaves, and during the following 

 spring grow new fructiferous filaments which would 

 propagate the disease afresh. 



Fructiferous filaments*. During the winter period of 

 vegetation the mycelium throws out fructiferous filaments 

 from the stomata of the under face of the leaves ; these may 

 grow in one night (Fig. 150 s). These filaments, projecting 

 from the stomata, number 4 to 8 in each stoma, and are 

 from i to mm. in height. 



Each ramification bears on its extremity 2 to 4 small 

 short points (sterigmata,}?\g. 150 ), to which is attached 

 the summer spore. 



* To examine fructiferous filaments with a scalpel, or better, with a razor ; a very small 

 fragment of the whitish fructification is detached from a glabrous leaf tangent to the 

 surface; it is placed on a slide and slightly moistened with methylated spirits. To get 

 rid of a 1 the air interposed between the filaments, a drop of chloride of calcium (50 per 

 cent, solution), or better, a drop of a liquid formed of equal parts of chloride of calcium 

 and glycerine with 50 per cent, of water is placed on it. If only a cursory examination 

 is required the fructifications are immersed in the water after being moistened with 

 methylated spirits. 



