Relations heiivccn Host ami Parasite. 6r 



aft'ect the one or the other injuriously or favorabh" will have a beannt>' on the 

 result. Thus, if the autumn is wet when the seed is sown nearly all the seeds 

 with spores attached will become infected, because the tissues under such 

 conditions remain longer soft and succulent. There are also limitations imposed 

 upon the germination of the spores, for they are not always capable of putting 

 forth their germinal tube at any given time even when moisture is supplied. 

 It is a general characteristic of smut fungi that they confine themselves to 

 particular hosts. This denotes a high degree of specialization in thus exer- 

 cising a selective power for their food, for there are other parasites which are 

 omnivorous in their tastes and show little or no preference in their selection 

 of hosts. 



Some of them such as the American corn smut utilize any young and 

 tender portion of the plant, be it leaf or stem or kernel of the cob, and when- 

 ever it is attacked do not at once proceed to use it up in its immature state. 

 The fungus stimulates the young and growing part to further growth, and 

 when it has multiplied excessively so as to form gall-like swellings, the food 

 stored up is rapidly appropriated and the spores are produced from a well 

 nourished and vigorously growing mycelium. Not only do they select a 

 host but in many cases they are only capable of infecting a particular stage 

 of that host, such as the seedling stage or the flowering stage. Wlien the 

 young seedling is infected, the parasite puts itself in touch with the growing 

 tissue and shares in all the growth and vigour of the plant until the criticaV 

 time arrives, when all the nutrient material prepared for the young embryo 

 is used up in the formation of smut spores. 



But the perfection of specialization is reached in the Carnation Smut fungus, 

 where the anthers of the flower are alone chosen for the production of the 

 spores. The parasite enters the plant through the young shoots as shown 

 by Hecke, and reaches the flower just before it is completely formed. There 

 it confines itself to the young stamens, using the anthers for the production 

 of spores and leaving the rest of the flower quite normal. It is the doctrine of 

 substitution carried to its utmost limits, for not only have the spores replaced 

 the pollen-grains, but they are carried by the fertilizing insects to other 

 flowers, and thus their distribution is assured as they are likely to be deposited 

 on the fruit which is being formed. This is an instance where the fungus 

 has selected the most favorable spot for the production of spores, and timed 

 it so that they are carried to their destination as if they Avere the flower's 

 oAvn pollen. There are degrees of proficiency in the parasite, just as there 

 are powers of resistance in the host, and this is the highest attainable in the 

 mode of infecting the most suitable part at the most favorable time. 



In considering the relations of the parasite to its host, we must take into 

 account all the phases of its life and distinguish between those which are 

 intimately bound up with the life of the individual plant and others which 

 ai'e passed outside of it. The mycelium of the fungus, for instance, ramifies 

 among the tissues and directly preys upon them, but there are portions of the 

 hyphfe detached at the surface for purposes of rapid propagation and known 

 as conidia, which are used to infect fresh plants. These conidia are known 

 to vary in their infective power, not only as we have seen, according to the 

 condition of the host, but dependent upon the nourishment they receive and 

 their age as well, or the generation to which they belong. If the mycehum 

 which produces them is well nourished, then as parts of the whole, they share 

 the increased vigour for a time, but with increasing age and the continued 

 absence of a host-plant tliey lose their capacity for infection. Biefeld^ has 

 shewn that these conidia can germinate in artificial nutritive solutions and 

 go on producing generation after generation of their kind. He cultivated one 

 form throuoh more than one hundred successive generations in the course of 



