ILLUSTRATIONS OF PHYTOPHTHORA INFESTANS. 415 



optical section and in plan) are turgid, and the chlorophyll- 

 corpuscles bright green and sharply defined. A fibro-vascular 

 bundle consisting of a few vessels, wood cells, and soft bast, 

 and surrounded by a sheath, has been cut across transversely ; 

 the palisade cells on its dorsal surface are arranged in a 

 peculiar manner, and a depression in the epidermis of the 

 upper side of the leaf corresponds with its course. To the 

 right hand of the section the parasitic mycelium of Phyto- 

 phthora infestans is rampant, though not as yet abundant 

 in this younger portion of the disease-spot. The tubular, non- 

 septate hyphse, are seen in the intercellular spaces, but not in 

 the cells themselves. Many of the cells are already losing 

 their turgescence and the chlorophyll-corpuscles are becoming 

 discoloured and misshapen, the protoplasm and cell walls 

 eventually turning brown and becoming disorganised. The 

 hyphae in the intercellular spaces send branches through the 

 storaata, especially on the lower side of the leaf, partly because 

 that is more sheltered, and there is more moisture, and these 

 branches become aerial hyphae which again ramify and produce 

 the conidia, as seen in the centre of the figure. The stoma to the 

 extreme right is cut through transversely, but not in the 

 median plane, and the tip of a hypha is already protruding 

 through it; one hypha in the lacuna below has been cut across 

 obliquely, and the extreme tip of another is just making its way 

 outwards towards the orifice of the stoma. The stoma to the 

 left of this is cut longitudinally, and the tips of two hyphae are 

 about to protrude through to the exterior; the stoma to the 

 left, again, is also in longitudinal section, and two branched 

 conidiophores have passed through and developed their conidia 

 outside. The details of these are similar to those in the 

 preceding figure. To the left of this stoma is one of the 

 numerous capitate hairs, with yellow, oily-looking contents; and 

 still further to the left one of the longer pointed hairs, the outer 

 cell walls of which are dotted with minute papillae. Similar 

 minute papillae exist on the outer walls of the epidermis-cells ; 

 the enlarged epidermis-cells surrounding the base of the hair 

 are often much larger than in the specimen drawn. A stray 



