THE FUNGI 283 



Infection of forest trees (Fig. 151), for instance, usually 

 takes place at points where wounds have been made by prun- 

 ing or where the bark has been injured by animals. In a similar 

 manner the infection of fruits is usually caused by bruising, 

 which breaks the epidermis and thus offers an opening to fungi, 

 whose spores germinate in the wound. It is for this reason that 

 shippers of apples and other fruits exercise the greatest care in 

 preparing the fruit for shipment. This is done by drying and 

 rubbing the surface of the fruit, by discarding infected speci- 

 mens, and by wrapping each fruit in paper to prevent contact 

 and infection from adjacent specimens. Many of these fruits 

 are -infected and rotted in part by molds like Penicillium 

 (Fig. 143), the spores of which are on the skin of the fruit 

 when it is packed. In other instances fungi infect host plants 

 by boring through epidermal walls through the agency of fer- 

 ments, which destroy the tissues ahead of the entering germ 

 tube. Infection by a filamentary fungus is often accompanied 

 by infections on the part of bacteria also, which complete the 

 processes of decay inaugurated by the higher fungous parasite. 



Invasion and disease production. The fungus hyphae, when 

 they have once penetrated into the interior tissues of a plant, 

 spread and destroy the tissues by the growth of a mycelium, 

 exactly as in the case of mold on bread (Fig. 140) or like a 

 tree parasite (Fig. 152). The invasion and breaking down of 

 the tissues is effected through the secretion of ferments by the 

 hyphse of the invading mycelium, just as the hyphae of Rhizopus 

 digest the starch in a slice of bread by the secretion of digestive 

 ferments. In this manner the mycelium of a parasitic fungus 

 may penetrate the hardest wood of trees, and then dissolve out 

 the wall substance and the stored food by a digestive process 

 which results in decay and death. Smuts and similar fungi, 

 which start to grow with the seed, keep pace in the above 

 manner with the growth of the seedling, following along the 

 paths of the least resistant tissues to places where the spores 

 are to be formed. 



This invading of the plant body, and the production of disease, 

 is, however, a much slower process in plants than the distribution 



