DISEASES OF FLOWERING PLANTS 197 



165. Effect on host plant. It should be understood 

 that fungi do not always destroy plant tissue. Their 

 presence often excites the protoplasmic contents of the 

 plant into abnormal activity, and rapid cell division occurs. 

 This produces various malformations and enlargements in 

 stem area. In other cases, the fungi exert an opposite 

 effect; cell growth becomes checked, and the plant is 

 dwarfed and stunted. The presence of fungous plants 

 may cause unusual development of the flower parts, the 

 petals and other floral organs becoming variously modified. 

 Changes in coloration of flower petals and foliage of plants 

 are often induced because of the presence of foreign plants. 

 The parasite may not kill the protoplasm of the host cell, 

 but it may so affect the chlorophyll bodies in the cell sap 

 that they are broken down, and the tissue of the leaf be- 

 comes somewhat colorless. 



In a larger part of the so-called diseases of plants the 

 mycelium of the parasite penetrates the vital part of the 

 plant, absorbing the cell wall and cell contents of the host. 

 As a result, the plant becomes weakened, or the complete 

 destruction of the structural tissue will result in the death 

 of the plant. Fungi affect plant tissue in the following 

 ways : 



(1) Killing of host cells by : 



(a) absorption of living cell contents. 



(6) absorption of cells or tissues. 



(c) killing of host cells and tissues by excretion of ferments. 



(2) Killing of organs or whole plants. 



(3) Shortening of life. 



(4) Premature development of buds. 



(5) Preservation of the host plant, and of host tissue. 



166. Character of infection. In attempting to dis- 

 cover the best methods of preventing attacks of parasites 

 on plants, the investigator must determine how the 



