ALGAL AND FUNGAL PROTISTS 41 



" The wall of Pythium is usually assumed to consist of 

 cellulose. However, as Trow has observed in the case of 

 P. ultimum, the cellulose reaction is often difficult to obtain. 

 The blueing with chloriodide of zinc is often faint or only 

 got after long treatment " (Butler). 



Phytophthora infestans is the cause of " blight," the 

 most serious disease of the potato. The leaflet issued by 

 the Ministry of Agriculture and Fisheries tells us that it was 

 first noticed in Europe and America in 1840 and that it 

 caused famine in Ireland in 1845. It is now always present 

 in the British Isles. The mode of entry into the host-plant 

 is shown in Fig. 12, a. The first sign of the disease is the 

 presence of brown or black patches on the leaves, which look 

 as if charred in places. 



At the margin of these places on the under side of the 

 leaf, and, in wet weather, on the upper side also, a white 

 powder is seen. This whiteness is the conidia, which are 

 borne on branched stalks pushed through the stomata from 

 within. Conidia germinate in the same way as those of 

 Cystopus, Fig. 8, 6, c. 



Details of the nuclear processes in a kindred species, 

 Phytophthora erythroseptica, which also destroys potatoes, 

 are given by P. A. Murphy (1918). The oogonium is peculiar 

 in that it pierces the antheridium to develope on its farther 

 side. Nuclear reductions occur leaving one nucleus in each 

 gametangium, these nuclei coalesce to form the single 

 nucleus of the oospore as in Pythium,. 



