xxxv.] POTATO DISEASE, I.-ITS ACTIVE STATE. 285 



see transparent threads running between the small 

 spherical leaf -cells ; these are the spawn -threads or 

 mycelium of the fungus. It should be especially noticed 

 that wherever the spawn touches the cells it discolours 

 them (as indicated by the darker shading), and causes 

 putrescence by contact. If we again look at the pallisade 

 cells near G, we observe that a spawn-thread has pushed 

 itself between them and between the cells of the upper 

 cuticle, and is emerging into the air. If We trace the 

 spawn-threads to the organ of transpiration at H, we 

 notice that a thread in its passage from the body of the 

 leaf has blocked up a so-called mouth. This choking 

 prevents the transpiration of vapour, and hastens pu- 

 trescence. Two other threads have pushed themselves 

 between the leaf -cells at G and A. When the larger of 

 the emerged threads is traced upwards to K, a treelike 

 growth is noticed; and this branching form is the fruit- 

 ing condition of the fungus of the potato disease called 

 Peronospora infestans, Mont. The name Peronospora has 

 been explained ; infestans needs no explanation. The 

 whole fungus is perfectly transparent, like colourless glass, 

 and extremely fine, thin, and attenuated in all its parts. 

 Some book illustrations give a very erroneous idea of the 

 fungus, owing to the use of thick lines and an unnatural 

 amount of dark shading. We notice as a rule that the 

 fruiting-stems or conidiophores, as at K, have comparatively 

 few joints or septa ; sometimes, however, old examples, as 

 at L, are full of joints. If we now look at the branches 

 MM, we observe that each is surmounted by a transparent 

 spore, technically termed (as in other species of Perono- 

 spora] a conidium as at NN; and to these bodies we 

 shall more specially refer further on. It must also be 

 noticed that all the branches are more or less constricted 

 or jointed in a peculiar manner, as at 00 ; and that each 

 joint has at one time carried a conidium, the lower conidia 

 having been pushed off as the branches have continued 

 their growth, as at PP. Sometimes : weakly ini- 



