66 BOTANY. 



CLASS II. Ascomycetes (SAC FUNGI). 



This class includes a very great number of common plants, 

 all resembling each other in producing spores in sacs (asci, 

 sing, ascus) that are usually oblong in shape, and each contain- 

 ing eight spores, although the number is not always the same. 

 Besides the spores formed in these sacs (ascospores), there are 

 other forms produced in various ways. 



There are two main divisions of the class, the first including 

 only a few forms, most of which are not likely to be met with 

 by the student. In these the spore sacs are borne directly 

 upon the filaments without any protective covering. The only 

 form that is at all common is a parasitic fungus (Exoascus) 

 that attacks peach-trees, causing the disease of the leaves 

 known as "curl." 



All of the common Ascomycetes belong to the second division, 

 and have the spore sacs contained in special structures called 

 spore fruits, that may reach a diameter of several centimetres 

 in a few cases, though ordinarily much smaller. 



Among the simpler members of this group are the mildews 

 (Perisporiacece) , mostly parasitic forms, living upon the leaves 

 and stems of flowering plants, sometimes causing serious injury 

 by their depredations. They form white or grayish downy 

 films on the surface of the plant, in certain stages looking like 

 hoar-frost. Being very common, they maybe readily obtained, 

 and are easily studied. One of the best species f>r study 

 (Podosphcera) grows abundantly on the leaves of the dan- 

 delion, especially when the plants are growing under unfa- 

 vorable conditions. The same species is also found on other 

 plants of the same family. It may be found at almost any 

 time during the summer; but for studying, the spore fruits 

 material should be collected in late summer or early autumn. 

 It at first appears as white, frost-like patches, growing dingier 

 as it becomes older, and careful scrutiny of the older specimens 



