Communications. 259 



Peronospora infestans, or Botrytls inftstans, two names (or one and 

 the same thing. The first name is the one most commonly used, 

 and is given to indicate the form of growth of the part bearing 

 the spores in the open air ; the other, from a fancied resemblance 

 to the grape cluster in the grouping of the spores themselves. The 

 first indication of the presence of the fungus is usually a change 

 of color in the leaves, a turning from a dark luxuriant, to a pale 

 green, and at length to a yellowish tint. On examining the 

 leaves closely, especially those near the ground, small spots, dark 

 brown or black, will be seen on their upper surface, with little 

 patches of white mould on the under side, right opposite the 

 spots mentioned. On examination of this mould with a micro- 

 scope of high power, it will be found to consist of a mass of very 

 slender tree-shaped stems, or thread?, often having four or five 

 branches, with a further subdivision of the branches into limbs 

 and twigs, the terminal points of which each bear an ovate- 

 shaped body, or vesicle. On tracing the downward extension of 

 these stems or threads they will be found to enter the leaf through 

 its breathing pores or stomata, and to be connected with larger 

 white threads within. These threads are termed mycelia (root- 

 like fibres) or spawn, and are found to extend throughout the 

 cellular structure of the leaf, to the stalk and even to the tubers, 

 crowding and matting the inter-cellular passages wherever they go. 

 Soon after these brown patches are seen on the leaves they also 

 appear on the stems, but the mould is rarely seen there. 



The oval-shaped bodies on the extremities are the fruit or seeds 

 of the fungus, and are termed spores or conidia. In the higher 

 orders of plants, which bear seeds for the reproduction of their 

 species and also in some of the cryptogams, some form or proc- 

 ess of fertilization is usually found, but nothing of the kind has 

 yet been discovered in the immediate development of these aerial 

 seeds of the P. mfestans, and they are therefore often called 

 asexaal spores to distinguish them from another form produced 

 in the same plant, in connection with which the fertilizing process 

 has been seen. Whether this fertilizing process is present in any 

 form in the Pero7iospora infestans has long been doubted, and is 

 still answered in the negative by some of the leading investiga- 



