280 



GENERAL MORPHOLOGY OF PLANTS 



host, give a rusty appearance to them, hence the name. The 

 mycelium is within the host and intercellular. It sends short 

 haustoria into the cells, where they absorb nutriment. In many 

 cases it causes hypertrophy of the host, i.e., stimulates the affected 



part of the host to ab- 

 normal growth, form- 

 ing witches' brooms in 

 one form on the bal- 

 sam, or galls as in the 

 case of the cedar apples 

 (paragraph 456), etc. 

 Some of these abnor- 

 mal growths are edi- 

 ble, as the branches 

 of an acacia in India 

 deformed by jEcidium 

 esculentum. and 



Fig. 244. 



Witches' broom on balsam (Abies balsamifera) caused 

 by a parasitic fungus (^cidial stage of Melampsorella 

 cerastii), from northern Michigan. 



in 



Scandinavia the 

 branches of the fir de- 

 formed by Mcidium 



corruscans. The life history is very complicated in some species 



and is well illustrated by the wheat rust. 



Wheat Rust (Puccinia Graminis}. 



446. The wheat rust produces one of the common rusts on 

 cereals and grasses in many parts of the world, and one stage in its 

 complete life history occurs on the barberry. 



447. The cluster-cup stage on the barberry. The diseased 

 spots on the leaves of the barberry are yellowish and round. 

 Upon the under side of the leaf there can be seen minute cup- 

 shaped structures (aecidia) distributed over the spots. These are 

 better seen with the aid of a pocket lens. In figures 248, 249, a 

 section of the barberry leaf through one of these spots shows its 

 relation to the host and the mycelium between the cells. The cup 

 is formed of a bundle of stout hyphae with stout short cells growing 

 out from this mycelium and bursting through the epidermis of the 



