THALLOPHYTES 



51 



looks like a brownish or blackish dot on the lilac leaf (Fig. 32) . 

 When this case is broken open, it is found to contain several 

 thin- walled sacs (sometimes only one), within which are 

 spores (Fig. 34). These heavy- walled cases, always bearing 

 characteristic " appendages " (Fig. 34), are the protected 

 structures that last through the winter, and it is their spores 

 that start new mycelia during the following spring. How 

 fertilization results in this case containing 

 sacs with spores, is not necessary for the 

 beginner to know. 



The three illustra- 

 tions given show how 

 the mycelium and the 

 structures it produces 

 are related to food- 

 supply as saprophytes 

 (as the molds) , external 

 parasites (as the pow- 

 dery mildews), or in- 

 ternal parasites (as the 

 downy mildews). 



The true Fungi are 

 so very numerous that 

 they cannot be pre- 

 sented in a brief ac- 

 count. It is impossible to give examples even of those 

 that are of great economic importance ; but the above illus- 

 trations will give some idea of the structure of the body and 

 its relations to the food-supply, and two other illustrations are 

 added because of their general interest. 



Wheat rust. The rusts are destructive parasitic Fungi 

 that attack very many plants, but public interest is chiefly 

 directed to those that attack the great cereal crops, chief 

 among which is wheat. The presence of rust in a wheat 

 field is noticed first by the appearance of reddish, rusty- 



FIG. 35. Summer spores of 

 the wheat rust, which form 

 the rusty lines on the 

 wheat ; notice the two 

 nuclei in each spore. 



FIG. 36. Winter 

 spores of the 

 wheat rust; 

 each spore has 

 two cells, and 

 each cell has 

 two nuclei. 



