STRUCTURE AND FUNCTIONS 29 



and human beings sometimes suffer from pulmonary 

 troubles which are caused by the growth of certain 

 fungi. 



Kingworm, scab, barber's itch, and similar skin 

 diseases of human beings are due to fungi. Sometimes 

 these diseases occur in other forms on lower animals, 

 from which they are readily transferred to human 

 beings who are working with them. 



Practically all parts of all plants are subject to 

 diseases. In some instances we have immediate and 

 complete destruction of the host, while in others death 

 may be very slow, and in others the plant may even 

 live and support the parasite indefinitely. 



The simplest form of parasitism is where the fungus 

 attacks and destroys a single cell without destroying 

 the surrounding cells. However, in some cases the 

 surrounding cells are stimulated to excessive growth, 

 and form small galls (see page 33). 



Most leaf-inhabiting fungi attack groups of cells, 

 which are destroyed, causing the characteristic leaf- 

 spots with which all agriculturists and horticulturists 

 are so familiar. 



Certain species of fungi cause the leaves to fall, and 

 thus prevent the plants from performing their regular 

 functions. The plant may die, or be stunted and 

 unable to produce fruit. 



Some fungi are known as "damping off" fungi, 

 because of their tendency to attack very young seedlings, 

 which they cause to die and fall over. These fungi 

 frequently come from the manures, on which they grow 

 very readily. After the death of the plant they con- 

 tinue to grow saprophytically on its decaying tissues, 

 and spread rapidly to neighbouring seedlings, which 

 are killed in turn. Older plants are not often attacked 

 by these organisms because of the development of the 

 protecting coats of cork and cuticle. These " damping 

 off" fungi are, no doubt, quite prevalent under natural 

 conditions, and sometimes become important as field 

 and garden pests, but are most destructive in seed beds. 



