48 



ELEMENTARY STUDIES IN BOTANY 



mon diseases on many vegetables. In this group the my- 

 celium lives upon a plant host and is a true parasite. It 

 does not spread upon the surface of the host, but penetrates 

 within it, crowding its way between the living cells of the host 

 (Fig. 29) . Thus it is not only a parasite, but also an internal 

 parasite. From its position against 

 the living cells of the host, the myce- 

 lium sends its haustoria through the 

 cell-walls (Fig. 29), and into these 

 haustoria the cell-sap of the proto- 

 plast enters, so that the protoplast 

 is dried out and dies. When a myce- 

 lium is living in this way in the interior 

 of a leaf, as a grape leaf, the drying 

 out and killing of the leaf-cells by the 

 haustoria is shown by the discolored 

 and finally brownish spots that ap- 

 pear on the leaves. 



FIG. 29. Downy mildew : branch of mycelium in 

 contact with two cells of a host plant, and send- 

 ing into them branching haustoria. After DE 

 BARY. 



FIG. 30. Downy mil- 

 dew: sporophores 

 emerging through the 

 "breathing pores" 

 of a leaf, branching, 

 and bearing spores ; 

 this form causes the 

 potato rot. After 

 STRASBURGER. 



Then the mycelium sends its sporophores to the surface of 

 the host (Fig. 30), for the spores must be formed where they 

 can be scattered ; and it is the sporophores coming to the 

 surface that represent the only part of trie parasite visible 

 outside the host. These spores are not formed within spo- 

 rangia, but are formed by cutting off the tip of the sporophore 



