DISEASES OF FLOWERING PLANTS 195 



forms the main working body. These filaments are the 

 mycelium. The mycelium .is in contact with its source 

 of food supply, known as the substratum. 



163. Reproduction. Parasitic fungi may be repro- 

 duced in three ways; vegetatively, by asexual spores or 

 by sexual spores. In vegetative reproduction, a mycelial 

 strand may be broken into several divisions. Under 

 favorable moisture and temperature conditions, each 

 division has the power of growth, increasing in size with 

 more or less rapidity. 



When reproduced by asexual spores, ascending branches 

 arise from the mycelial tissue. From the extremities of 

 these branches, independent cells are separated. These 

 cells have the power of producing rapidly another mycelial 

 strand if conditions favor it. They retain their vitality 

 for a short period only, and therefore function in an im- 

 mediate reproduction of the species. The branches which 

 produce these spores, are called ascending hyphse or sporo- 

 phores. Sometimes, asexual spores are formed within 

 the parent cell, and are liberated by the bursting or 

 decay of the cell wall. 



Some fungous plants have cells which function as sex 

 organs. They are called the oogonia or egg cells, and 

 the antheridia or sperm cells. When conditions are 

 favorable, the contents of the antheridium fuse with 

 the oogonium, and an egg cell results. These egg cells 

 often retain their vitality during long periods, and serve 

 to carry the disease through a period unsuited for growth 

 of the mycelium. 



Sometimes, especially in the case of parasites, special 

 branches are formed which penetrate the host cells and 

 absorb the food material. These special, absorbing 

 branches are called haustoria, meaning absorbers. 



