100 



PLANT GROWTH 



organisms as aphids that suck the plant juices. Since dis- 

 eases are caused by microscopic organisms, often inside the 

 host plant, it is impossible to see the early stages of injury 

 and difficult to make a close association of the injury and 

 the organism causing it. 



The injury may be caused by the fungus growth, called 

 haustoria, which may enter into the cell or grow between the 



Fig. 17. Cross section of a radish leaf infected with Albugo Candida. The inter- 

 cellular mycelium, m, produces the haustoria, h, in the mesophyll cells, s. The conidia, 

 c, are reproduced in chains from the conidiophores, x, and escape after the rupturing of 

 the epidermis, e. (From Melhus and Kent's Elements of Plant Pathology. By permis- 

 sion of The Macmillan Co., publishers, 1939.) 



cells. The infection of the radish leaf (Fig. 17) shows both 

 types. In some cases the fungus secretes enzymes to digest 

 plant material, and in other cases it absorbs material already 

 in solution. After the fungus has penetrated a large area of 

 the host tissue and can absorb large quantities of food it 

 forms great masses of spores. In the above figure they are 

 borne in chains but many other arrangements are known for 

 other diseases. A single spore from such a mass can cause 

 another plant to become infected. A new generation of 



