Reproductive Organs. 9 



In other species there is considerable variation. According to Woronin 1 , 

 the teleutospores of P. helianthi germinated equally well when kept dry in a 

 room, or when taken from the leaves of a plant which had been under the 

 snow all winter; and Carleton 3 found them to germinate even without a 

 resting period. 



After wintering, Eriksson found that the teleutospores of P. graminis 

 could retain their germinating power even to the beginning of autumn 

 September in Sweden. 



In Victoria the germination of the teleutospores of P. graminis has been 

 tested for several seasons, and it is found that after a period of rest ex- 

 tending over about eight months, and including the usually hot and dry 

 summer, they begin to germinate in our spring (September), and if the 

 weather conditions are favourable, may retain their germinating power 

 until November. Rusted straw has been kept outside exposed to the 

 weather, and in a room, but no germination took place outside of the above 

 months. When spores have once begun to germinate it is a mistake to 

 suppose that they will all germinate at the same time, for there is a certain 

 irregularity about germination in the open which causes the period of ger- 

 mination to be considerably extended. 



INFECTION. 



The mode of infection usually varies in different classes of spores. The 

 germ tube of the uredospore or aecidiospore generally enters by the stomata 

 into the tissues of the host-plant, while that of the sporidiolum bores through 

 the epidermis direct, and this difference in the mode of infection influences 

 the stage at which it occurs in the development of the host-plant. The 

 sporidiola are, as a rule, only able to infect young and tender portions of 

 the plant; but the uredospores and aecidiospores can evidently attack old 

 and fully developed organs. 



Further, since the germ -tube of the sporidiolum) in piercing the epider- 

 mal cells must dissolve the wall at the spot where it enters, there is evidently 

 a reciprocal influence exerted between the parasite and its host; but even in 

 the case of the uredospores and aecidiospores, where an entrance is effected 

 through the stomata, there also exists some mutual relation, for it is only 

 in certain hosts that the parasite can grow and develop. 



This relation will be more fully dealt with when the origin of para- 

 sitism is considered, but meanwhile the fact may be emphasized, that the 

 mere entrance of the germ-tube through the stoma does not constitute 

 infection. Miss Gibson 1 carried out infection experiments with uredo- 

 spores and aecidiospores of various rusts on a number of plants other than 

 the original hosts, and she found that while the germ-tube may enter the 

 stoma freely, yet, once inside, death ensues sooner or later, and in no single 

 instance was a haustorium formed. The fungus was unable to penetrate 

 the cells of the foreign host-plant, and so died of starvation. 



The period which elapses between the entrance of the germ-tube into 

 the host-plant and the appearance of the result is known as the incubation 

 period. During this time the germ-tube grows and ramifies among the 

 tissues, abstracting nourishment from the cells by means of haustoria, and 

 finally proceeds to the formation of fresh spores. The first visible trace of 

 this does not generally appear before eight days, but, according to the 

 nature of the fungus, it may take much longer. Schimper, in his 1 masterly 

 work on Plant Geography, begins with the statement " No factor affecting 

 plant life is so thoroughly clear as the influence of water," and even^ for 

 parasitic fungi this factor is of prime importance. Smith l has determined 

 the water relation for Pucdnia on asparagus, and probably the same prin- 

 ciple applies in a general way to other rusts. From a study of the direct 



