192 



PLANT BIOLOGY 



packed in the perithecia. They do not ripen in the autumn, but 

 fall to the ground with the leaf, and there remain securely pro- 

 tected among the dead foliage. The following spring they mature 

 and are liberated by the decay of the perithecia. They are then 

 ready to attack the unfolding leaves of the willow and repeat the 

 work of the summer before. 



The wheat rust. The development of some of the rusts, as the 

 common wheat rust (Pitccinia graminis), is even more interesting 



and complicated than that of the 

 mildews. Wheat rust is also a true 

 parasite, affecting wheat and a few 

 other grasses. The mycelium here 

 cannot be seen by the unaided eye, 

 for it consists^ of threads which are 

 present within the host plant, mostly 

 in the intercellular spaces. These 

 threads also send short branches, or 

 haustoria (Fig. 132), into the neigh- 

 boring cells to absorb nutriment. 



The re sting- spores of wheat rust 

 are produced in late summer, when 

 they may be found in black lines 

 breaking through the epidermis of 

 the wheat stalk (black-rust stage). 

 They are formed in masses, called 

 sort (Fig. 280), from the ends of 

 numerous crowded mycelial strands just beneath the epidermis of 

 the host. The individual spores are very small and can be well 

 studied only with a microscope of high power 

 (X about 400). They are brown two-celled bod- 

 ies with a thick wall (Fig. 281). Since they are 

 the resting or winter-spores, they are termed teleu- 

 tospores ("completed spores"). Usually they do 

 not fall, but remain in the sori during winter. 

 The following spring each cell of the teleutospore 

 puts forth a rather stout thread, which does not 

 grow more than several times the length of the 

 spore and terminates in a blunt extremity. This 

 germ tube, promycelium, now becomes divided 

 into four cells by cross walls, which are formed 

 from the top downwards. Each cell gives rise to a short, pointed 

 branch which, in the course of a few hours, forms at its summit 

 a single spore called a sporidium. This in turn germinates and 

 produces a mycelium. In Fig. 282 a germinating teleutospore 

 is drawn to show the promycelium, p, divided into four cells, 



FIG. 280. SORI CON- 

 TAINING TELEUTO- 

 SPORES OF WHEAT 

 RUST. 



FIG. 281. TE- 

 LEUTOSPORE 

 OF WHEAT 

 RUST. 



