440 Prof. H. Marshall Ward. The Relations between 



part, structures which are already dead, and so interfere with the 

 transpiration current, and other large groups of functions, more by 

 the mechanical injury done than by direct injury to living cells.* 



In the group of the Ustilaginece we have some of the most remark- 

 able parasites known, and the relation of the host-plants (chiefly 

 species of Gramineie and Cyperaceee) to them must be very different 

 in detail. 



Fig. 15. Zea metis. Portion of inflorescence (reduced) with malformations pro- 

 duced by the parasitism of the fungus Ustilago Maidis (Sorauer). 



In the more typical cases we find that the sporidia or conidia ger- 

 minate in artificial nutritive media, and go on producing generation 

 after generation of their like,f and this undoubtedly occurs in the 

 open fields, &c. Brefeld states that he has cultivated one form 

 through more than a hundred successive cultures in the course of a 

 whole year, and that this corresponds to about 1500 successive sprout- 

 series or generations, } but towards the end of the period the germi- 

 nating power of the successive conidia became weaker and weaker, 

 and at last failed. 



* The germinal hyphse developed from the spores of such fungi often find their 

 way into the wood, cambium, &c, by means of wounds, caused by mechanical 

 breakage, the nibbling of mice, squirrels, the punctures of insects, frost-cracks, the 

 blows of hailstones, and so forth, which introduce us to a different aspect of the 

 relations between host and parasite. 



t Brefeld, ' Bot. Unters. Tiber Hefenpilze,' part 5, Leipzig, 1883. 



X Brefeld, in a lecture before the Klub der Landwirthe zu Berlin, 1888 (' Nach- 

 richten aus d. Klub d. Landw. zu Berlin,' 1888, Nos. 220—222). 



