PENETRATION THROUGH WOUNDS 251 



graminis tritici. When he inoculated 7-dav-old wheat seedlings 

 with urediniospores within the range optimum for germination, 

 he found that maximum infection required at least 36 hours | Pel- 

 tier (1925)]. This period was determined by use of a series of 

 plants inoculated at the same time by a suspension of uredinio- 

 spores. After definite intervals the surfaces of some of the plants 

 were permitted to become dry. An arbitrary scale to show 

 severity of infection was then employed as a basis of comparison. 

 Leaves on which 5 or fewer rust pustules developed were regarded 

 as in class 1; those with 6 to 10, in class 2; and those with 1 1 to 25, 

 in class 3. Certain of Peltier's data are shown in Table 25. Ap- 



TABLE 25 



Time Required for Infection by Puccina graminis tritici 



Definite Intervals Plants Severity of 



after Which Leaves Infected Infection 



Dried {hours) {percentage) {class) 



2 



3 1.7 1 

 6 17.0 1 



12 28.0 1 



16 33.0 1 



20 59.0 1 



24 89.0 2 



30 98.0 2 



36 100.0 3 



parentlv the minimum time required by the wheat stem-rust 



fungus for actual entrance through the stomatal aoerture is he- 

 ir ^ 



tween 2 and 3 hours. 



PENETRATION THROUGH WOUNDS 



There is a large group of facultative parasites that lack ability 

 to produce disease or decay in^ intact tissues but can establish 

 themselves in wounds and thence spread to normal tissues. The 

 heartwood and sapwood rots of trees are notable in this respect. 

 Many of these fungi gain entrance through branch stubs or scars 

 left in pruning, through fire scars, through abrasions from contact 

 of limbs, or through injuries by other fungi, insects, or rodents. 



Little is known about the fundamental differences between these 

 so-called facultative parasites and true parasites or about actual 



