2 THE FUNGI WHICH CAUSE PLANT DISEASE 



when ripe, a resting seed, the cells of the potato tuber in winter, 

 are undoubtedly alive, yet their activity is so little that many 

 organisms can gain a foothold upon these stages of the plant 

 that cannot do so at more vigorous periods of their exist- 

 ence. 



Tubeuf '^ ranks as hemi-parasites those organisms that usually 

 are parasites, but may sometimes become saprophytic, and as 

 hemi-saprophytes such as are usually parasitic, but may excep- 

 tionally become saprophytic. These distinctions are of little 

 import, other than to bring out clearly that each species has its 

 own limits as to food requirements. 



It is hardly to be thought that these parasites and saprophytes 

 have always been dependent organisms. The true fungi for ex- 

 ample are best to be regarded as degraded descendants of algee, 

 in which ancestors they once possessed chlorophyll and could 

 prepare their own food from mineral matter by the aid of sun- 

 light. 



No discussion of the general metabolic processes of the fungi is 

 here necessary further than to indicate that among the products 

 of their activity there are various excretions and secretions, which 

 bear important relations to parasitism. Thus certain fungi grow- 

 ing in artificial culture produce enzjrmes or organic ferments 

 capable of softening and dissolving cellulose, also toxins, poisons 

 which are capable of killing the cells of the host plant. Such 

 enzymes and toxins are numerous and their bearing upon par- 

 asitism is obvious. Th^ enable the parasite to kill adjacent cells 

 of the host and then to effect an entrance through the cell walls 

 to the protoplasm and other nutrients contained within the 

 cell. 



The presence of the parasite, or secretions produced by it, often 

 calls forth abnormal growth responses from the host. These take 

 very diverse forms, either the undergrowth or overgrowth, hyper- 

 trophy, of single cells or tissues, or even the excessive development 

 of large plant parts as in the case of the witches' brooms, and the 

 "double flowering" of the dewberry. 



The probable relations of the groups under consideration to the 

 other members of the Thallophyta are suggested in the following 

 scheme.** 



