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fungus to spread with amazing- rapidity ; when mature 

 they are easily detached, and being extremely small the 

 slightest current of air carries them in large numbers to 

 the leaves of adjoining plants. The conidia are only 

 produced when the air is moist and sultry. Alighting 

 upon the healthy leaf of the potato, under such atmospheric 

 conditions they germinate in one of tw^o ways. Either, 

 each conidium behaves as a spore-case and liberates motile 

 spores, or it germinates by sending out a hne filament. 

 By the former method it opens at the tip and liberates 

 about eight small motile spores w^hich swim about vigor- 

 ously in the thin film of water on the surface of the leaf. 

 As in the case of Fythium, the fungus causing " Dampmg- 

 off " of seedlings, these motile spores are each furnished 

 with two hairs of protoplasm, by means of which they are 

 enabled to move m the water. After a short period the 

 motile spores come to rest, withdraw the minute hairs and 

 give rise to a short filament which either grows through 

 one of the stomata into the leaf, or dissolves a way 

 through the outer skin of the leaf, and so starts a disease 

 spot. In other cases the conidium simply sends out a fine 

 tube and produces an infection by the same methods as 

 do the motile spores. After infection the fungus, feeding 

 on the cells of the host, grows rapidly, in the form of 

 branched threads, within the tissues of the leaf. The 

 conidia are extremely small, being less than one- 

 thousandth of an inch in length. Enormous numbers are 

 produced from a single diseased area of a leaf, and, 

 further, under suitable conditions the fungus grows and 

 reproduces itself very rapidly. It is therefore easy to see 

 why, in w^eather conditions favourable to its growth, the 

 fungus spreads in a few days over whole fields of potatoes. 



In the earlier chapters it has been shown that a leaf, 

 such as that of the potato, is really a complicated machine 

 for the manufacture of food material. Many of the cells 

 contain chlorophyll, the green colouring matter w^hich, in 

 the presence of sunlight, utilises carbon-di-oxide obtained 

 from the air to produce organic chemical compounds like 

 sugar and starch. The complex organic substances 

 elaborated in the leaves are passed on to the stem and 

 ultimately stored m the potato tubers in the form of starch. 

 Thus it is obvious that by injuring the leaves, the potato 

 disease reduces the weight of tubers obtained from a given 

 plant, and the earlier the attack occurs in the season, the 



