ILLUSTRATIONS 



PLATES 

 Historical Relations op Sugar-Beet Seedlings and Phoma betae 



Page 

 Plate I. Fig. i.— Section of a sugar-beet seedling invaded by Phoma betae, 

 showing distribution of the mycelium and the action of the fungus on the 

 protoplasm and cell walls. Fig. 2. — Section of sugar-beet seedling show- 

 ing characteristic action of Phoma betae on the cyptoplasm and nuclei and 

 cell walls in cases of serious infection. Fig. 3. — Section of sugar-beet seed- 

 ling, showing Phoma betae penetrating the cell walls and expanding in one of 

 the cells. Fig. 4, 5, 6. — Abnormal nuclei from uninfected cells adjacent 



to invaded tissue of sugar-beet seedlings 58 



Plate II. Fig. i. — Section through a sugar-beet seedling which has recovered 

 from an attack of Phoma betae, showing a }'oung pycnidium of the fungus 

 forming on the discarded, killed tissue. Fig. 2. — Longitudinal section 

 through a sugar-beet seedling which had recovered from an attack of root 

 sickness due to Poma betae, showing the presence of the fungus established 

 in a condition of reduced virulence in the living cells 58 



Perennial Mycelium in Species of Peronosporaceae Related to 

 Phytophthora infestans 



Plate III. Fig. i. — Cystopus candidus on Lepidium uirginicum. Fig. 2. — A, 

 The two leaves at the left show the amount of sporulation of Peronospora 

 parasitica on leaves of Lepidium, virginicum; B, the two leaves at the right 

 show Cystopus candidus fruiting on leaves of Capsella bursa pastoris. Fig. 

 3. — Peronospora viciae on Vicia sepium, 70 



Hibernation of Phytophthora infestans in the Irish Potato 



Plate IV. Phytophthora infestans: Infection of potato tubers. Fig. i. — 

 Cross section of a tuber which was infected with P. infestans and was 

 planted in the greenhouse in rather dry soil. Fig. 2 .—This tuber was inoc- 

 ulated at the eye surrounded by the paraffin ring. Fig. 3. — Cross section of 

 an infected tuber planted in sterilized soil in the greenhouse which devel- 

 oped a shoot that became infected through the parent tuber. Fig. 4. — The 

 small stunted shoot which grew from this infected tuber shows the pro- 

 gressive discoloration caused by P. infestans growing up the stem 102 



Plate V. Phytopththora infestans: Infection of a potato plant 102 



Plate VI. Phytophthora infestans: Infection of potato shoots and plantlets. 

 Fig. I.— This shoot grew from a diseased tuber planted in the greenhouse 

 under field conditions. Fig. 2. — This shoot, which had not reached the 

 surface of the soil, grew from an infected tuber in the field. Fig. 3. — This 

 plantlet was the progeny of a diseased tuber planted in the open 102 



Plate VII. Phytophthora infestans: Infection of potato plants. Fig. i. — A hill 

 of potatoes having 13 shoots grown from a whole infected tuber in the field. 

 Fig. 2. — In this hill with two shoots the fungus has reached the surface 

 and killed its host. Fig. 3. — This shows the hill illustrated in figure 2, in 

 its position in the row where it grew 102 



Plate VIII. Phytophthora infestans: Infection of potato plots. Fig. i. — A 

 comer of the plots where infected seed potatoes were planted. Fig. 2. — 

 The area within the white lines shows a spot where infection is much more 



prevalent than in the surrounding plants 103 



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