PLANT DISEASES 



251 



the green color due to chlorophyll. Fungi take their food 

 from the substance in or on which the mycelium is grow- 

 ing, by diffusion of the soluble substances through the 

 cell-walls and protoplasmic lining of the mycelium. Many 

 fungi secrete enzymes that 

 dissolve cellulose and 

 other substances, making 

 them available for ab- 

 sorption; these secretions 

 often kill the protoplasm 

 of the host, thus compel- 

 ling it to give up nutri- 

 tious solutions to the 

 parasite. Others send spe- 

 cialized branches of myce- 

 lium (haustoria) into the 

 host cells. These absorb 

 the food substances that 

 come to these cells. 

 Eventually, they cause the 

 death of the host cells. Sometimes the irritation of the 

 parasite causes a response on the part of the host in the 

 form of knots, swellings, etc. A good example of this is 

 seen in the black-knot of plums and cherries. 



During their vegetative stage, fungi multiply by means 

 of various kinds of asexual spores cut off from the myce- 

 lium. This method of reproduction corresponds to the 

 multiplication by sprouts, sets, bulbs, etc., of the higher 

 green plants. Many fungi are also known to form sexual 

 spores called oospores, ascospores or basidiospores, accord- 

 ing to the group in which they occur. These sexual spores 



FIG. 129. The bread mold fungus. 

 (Whetzel) 



