TYPES OF CRYPTOGAMS; THALLOPHYTES 263 



THE STUDY OF MICROSPH^RA 



314. Occurrence. Species of Microsphcera and allied forms 

 occur in late summer and fall on leaves of various herbaceous and 

 woody plants. The growth is confined to the surfaces of the leaf 

 (upper, lower, or both). Among the most available species are 

 those which grow upon lilac, oak, grape, cherry, willow, and wild 

 plants of the sunflower family. Some species are known to occur 

 on only one host-plant, others occur on several or a large number, 

 and the host-plants may belong to one or more than one family. 



Besides Microsphcera there are about five other genera, any of 

 which may be substituted or studied comparatively. They are dis- 

 tinguished by the form of the appendages, together with the number 

 of spore-sacs (asci) in each sac-receptacle or perithecium. 



The species of fungi which Microsphcera represents are called 

 powdery mildews. 



With naked eye and magnifying glass examine the surface of a 

 leaf bearing powdery mildew. Note which surface and what portion 

 of the surface is occupied by the fungus, whether the occupied area 

 is restricted or not, the color, and any other characters. 



315. Examination with the Microscope. Place a small drop of 

 water on the leaf where the fungus occurs, if possible where dark- 

 colored specks occur among the mycelium. Pick from the leaf a 

 portion of the fungus loosened by the water and place with a drop 

 of water on a slide. Place a cover-glass over it. Examine under 

 a power of about fifty diameters. The dark-colored specks will be 

 seen as somewhat spherical bodies (perithecia). Note their structure 

 and color and their appendages. Have the perithecia any regular 

 way of opening? Note the length of the appendages as compared 

 with the diameter of the perithecia ; also note the form of the tips 

 and of the base, the color and any variation of color in different 

 parts of the appendages. Keep the left hand on the focusing screw, 

 and with the needle in the right hand press with gentle but varying 

 stress upon the cover-glass to rupture the perithecia. Even with 

 great care broken cover-glasses may result, but this pressure should 

 force out the contents of the perithecia. Another method is to 

 remove the slide from the microscope and, with a pencil rubber 



