520 KEw JKHsEv A(4Ki('ri;rrKAL collegp: 



scope is required that is provided witli a measurer (mieronieter). by 

 means of which the average size of the penthicia is determined — 66 

 to lo.") mieromillimetres* in diameter. The numher of the spore sacs 

 Avithin the penthicium — 3 to S — is determined and their shape globose 

 and size 42-70 by o2-50u. Xext the spores, 4-8 in each sac, are 

 measured (18-23 by L>-12»)- ^u'^fl then attention is directed to the 

 numlK'r, position and size of the appendages or holdfasts of the pen- 

 thicium, particttlar account being taken of the unusually complex 

 forking at the tips. When specimens agree with the description sug- 

 gested in the above outline they are considered as belonging to the 

 genus Microspha?ra, and the species Microsphcera AIni (Wallr. ). The 

 generic name Microsph?era means little sphere, as suggested l^y the 

 shape of the minute penthicia, and the specific addition Alni is from 

 Alnus, the Latin for alder, upon various species of which the mildew 

 is common. 



It might be thought an easy matter to determine the species di- 

 rectly from a known list of the plants upon which it has been found. 

 This method might answer the purpose, provided that only one kind 

 of mildew infested any grown host. For example, the oaks are gen- 

 erally attacked by the Microsphnm Alni (Wallr. ), Imt also by three 

 other species, and two varieties of the one in question. The only 

 safe way is to give each specimen of mildew a searching microscopic 

 examination. 



Their Development. — Unlike the rusts, smuts and various deeply- 

 seated blights, the powdery mildews, as stated before, are su]ierficial 

 and Iheir growth is easily watched. Starting at a point where the 

 spore has fallen, the mildew is first noticed as a speck, and this rapidly 

 enlarges to the size of a dime, retaining its circular form unless meet- 

 ing with the rim of another advancing colony when irregular shapes 

 are produced. The spreading may continue until the whole infested 

 area, whether leaf or stem, is coated over. It is frequently easy io 

 see without a glass the radiating lines which converge here and there 

 in the points of original attack. In like manner the spore cases may 

 be seen formed along the main threads or "spokes" and giving to 

 the coating of mildew something of a starry appearance. Some of 

 these characteristics may l)e seen in the representations in Plate XII. 



The rate with which the mildew develops depends upon the kind 

 of mildew, of host and the surroundings of heat and moisture. Upon 



*A micromillimeter designated by the Greek letter )/ — mew — is one twenty-five 

 housandth (2 j^o o) o^ ^^ inch. 



