414 H. MAESHALL WARD. 



out, any theoretical matters touched upon being referred to 

 incidentally and in connection with points of structure, &c., in 

 question. It has not been my intention to discuss hypothe- 

 tical matters at any length, and hence the form of the descrip- 

 tive letterpress. 



Fig. 1 was drawn from a preparation made by cutting off the 

 epidermis of the leaf at the margin of a disease-spot, where the 

 tissues are as yet green, and the white aerial hyphae of the 

 fungus form the well-known velvety " bloom." Two stomata 

 are shown, and a number of intervening cells of the epidermis. 

 The stomata are very wide open, and each guard-cell contains 

 chlorophyll-corpuscles and a nucleus (see the stoma on the 

 right). The vertical boundary walls of the epidermis-cells are 

 sinuous. From each stoma emerge branched aerial hyphge of 

 Phytophthora infestans, and their continuation into the 

 leaf can be traced for a short distance. The aerial hyphae 

 branch considerably and produce the ovoid " conidia " at the 

 tips of the branches, the tip often growing forwards and deve- 

 loping another ''conidium'^ before the first one has fallen^ 

 and so on repeatedly, as indicated by the peculiar joint-like 

 swellings (see fig. 3 for further explanation of this process). 

 Septa are found here and there below the joint-like swel- 

 lings ; otherwise the hyphae are not segmented. Conidia 

 in various stages are still attached to the hyphse, others 

 have fallen or been knocked off during manipulation ; such 

 a fallen conidium is seen lying on the epidermis. The pre- 

 paration was made from a perfectly fresh leaf, and examined 

 in water. 



Fig. 2. — This transverse section of a potato leaf was made 

 across a similar part of the disease-spot, but passed somewhat 

 closer to the dark brown part of the spot, where the cells are 

 already dead ; this is evident at the right hand of the drawing. 

 The epidermis is very similar on both sides, the stomata being 

 less numerous on the proper upper surface of the leaf. The 

 mesophyll to the left of the preparation is still quite healthy 

 and free from the mycelium of the fungus ; the cells (seen in 



