80 PHYSIOLOGICAL RELATIONS 



rot of sugar beets may cause damping-off diseases of seedlings, as 

 well as other diseases, in several different families of hosts. Of 

 the numerous cases which might be cited in this connection, many 

 are in need of critical study, notably Exoascus. Among hemi- 

 parasites, or facultative parasites, such studies will doubtless lead 

 to a considerable reduction in the number of the so-called species 

 of such fungi. On the other hand, infection experiments have 

 compelled mycologists to break up among others the old species 

 Puccinia graminis into several forms, frequently termed biological 

 or physiological forms, or subspecies, which, in some cases, are 

 entirely indistinguishable one from another on purely morpho- 

 logical grounds. Each form has a restricted number of host plants, 

 and it is believed that no cross infections occur. Many similar 

 cases have been clearly demonstrated for the Uredinales. 



It has recently been shown that certain mildews, notably Erysiphe 

 graminis, may likewise be broken up into forms restricted each to 

 one or more host plants. The two fungi mentioned are instances 

 where each parasite, as a species, is capable of infecting a large 

 number of host plants. It remains to be seen to what extent such 

 differentiation of forms is to be found in species more restricted 

 as to host plants. 



4. Experimental evidence was required to demonstrate the long- 

 suspected connection between Puccinia graminis, the grain rust, 

 and the common aecidium on the barberry, AZcidium Berberidis. 

 Those experiments, although preceded by studies in hetercecism 

 upon Gymnosporangium, mark a very distinct epoch in infection 

 work, for hetercecism has proven a very important biological phe- 

 nomenon. Within the past few years, particularly, much has been 

 done towards a systematic endeavor to connect by experimental 

 proof the hetercecious forms of Uredinales. Nevertheless, much 

 valuable work remains to be done, and the observant student will 

 constantly find suggestions in the proximity of host plants taken 

 in connection with the sequence of stages in these fungi. It is 

 well known that the occurrence of a uredo or teluto stage in con- 

 nection with an secidium, or closely following the latter, is not the 

 final proof that these stages are connected. A close observation of 

 many affected host plants during different seasons may, however, 

 give some valuable clews as to relationships and prevent fruitless 



