SEED IMPROVEMENT MEETING. 205 



become affected by a disease even wben exposed to it, just as 

 some people are immune from an attack of measles, chicken 

 pox, or yellow fever. I am pleased to announce that there is 

 on record a number of cases of plants resistant to destructive 

 diseases, that have been obtained by selection and by hybridiza- 

 tion, and that these plants have apparently transmitted their 

 resistant qualities to their progeny in succeeding years. 



Before beginning a discussion of the methods by which 

 resistant plants may be developed it will be well to discuss 

 briefly the nature of fungi and bacteria and to point out thg- 

 relationship which the parasitic forms bear to their host plants. 



Fungi and bacteria are plants just as are the higher forms 

 upon which they live. They are very much reduced in size 

 and in most cases in their physiological activities because they 

 are not capable of manufacturing their own food as the 

 green plants do but live upon organic substances already 

 produced. Bacteria are very small, simple, one celled organisms- 

 that increase by simple division of the body. They absorb 

 directly through their walls the soluble food necessary for 

 their growth and development. Fungi are characterized, for 

 the most part, by having a vegetative body composed of thread- 

 like strands called mycelium. Fruit bodies of various forms 

 are produced by the different species and these bear or con- 

 tain the spores or reproductive bodies of the fungus. These 

 spores are distributed by wind, water, and other agents and 

 upon germination reproduce the vegetative stage of the fungus. 



Those forms of bacteria and fungi that live upon dead 

 organic matter are called saprophytes and those that prey upon 

 living tissue are known as parasites. The plant sustaining 

 the parasite is regarded as the host. Some forms ordinarily 

 saprophytic may become parasites if the plant attacked becomes 

 weakened and conditions are particularly favorable for the 

 development of the parasite. Such forms have been named 

 facultative parasites. 



It is, no doubt, well known to you that plant as well as 

 animal tissue is made up of cells composed of protoplasm or 

 living matter containing various organic and inorganic sub- 

 stances in solution or as solid particles. In most plants the 

 protoplasm is surrounded by a wall of cellulose. An aggre- 

 gation of these little box-like cells makes up a tissue which, in 



