117 



Septa Differences of means of septa 



H. Nos. - 1 and 3 0.27 .16 



H. Nos. 1 and 15 0.80 =*= .12 



H. Nos. land 16... 0.58 .11 



H. Nos. 15 and 16 0.22 == .11 



It is to be noted that the differences between Nos. 15 and 16 are quite 

 as large relative to the probable error as are the differences between Nos. 1 

 and 3. Ravn (91) , speaking of three species of Helminthosporium, says that 

 the septa are very variable, and that specific differences can not be derived 

 from them. Very abnormal septation was frequent; for example, on green- 

 wheat agar (Fig. 14) and other uncongenial media. 



FIG. 14. Various abnormal conidia of 

 H. No. 1 as grown on green-wheat agar. 



Conidial shape. The shape of the conidia, together with their size and 

 septation, are in the genus Helminthosporium the three most-used charac- 

 ters in description. Indeed in published descriptions of many species these 

 are the only important characters mentioned, and often one or more of these 

 is lacking. Conidial shape in certain species is very characteristic, partic- 

 ularly in H. inaequalis, H. geniculattim, H. ravenelii, and also in my H. 

 No. 29. Much stress has been laid on conidial shape as a means of distin- 

 guishing certain cereal Helminthosporiums, particularly in distinguishing 

 H. sativum from //. teres. 



Merely to look at two lots of conidia with the microscope, even with 

 the aid of a comparison ocular, is not a satisfactory means of ascertaining 

 the prevailing conidial shape. Many strains of Helminthosporium vary 

 greatly as to conidial shape, and conidia of one shape are mixed with those 

 of another (PI. XIX XXI). The important question is, what is the 

 relative frequency of the various shapes? But before any fair estimate of 

 this can be made, standards must be established as to what are the essen- 

 tial characters of the various shapes. 



One factor of preponderating influence in determining these conidial 

 shapes is the position that the point of greatest diameter occupies on the 



