4 BULLETIN 981, U. S. DEPARTMENT OF AGRICULTURE. 



branches, and no rootstocks. Johnson grass (Andropogon halepen- 

 sis), on the other hand, is a perennial under favorable conditions, 

 with stems more slender than those of Sudan grass, 3 to 4 feet high, 

 few, narrow, rather harsh leaves which have thick white midribs, 

 loose, open, often drooping panicles, few to many tillers, branching 

 somewhat after maturity, and with numerous aggressive rootstocks 

 which make it difficult to eradicate from cultivated fields. The 

 seed characters of the two grasses will be considered in detail under 

 "Seed production." 



According to the classification of Piper in his " Forage Plants 

 and Their Culture," Sudan grass belongs to his proposed new agro- 

 nomic group called "grass sorghums," and its technical name is 

 Andropogon sorghum sudanensis (19, pp. 33-34) 3 and not Andropogon 

 halepensis, under which name it was obtained from Africa. 4 The 

 dividing line botanically between Andropogon sorghum and Andro- 

 pogon halepensis has been determined by Piper as the presence or 

 absence of rhizomes, or rootstocks. This characteristic provides a 

 very definite line of demarcation, and a study of the map of Africa 

 and the Mediterranean region of Europe and Asia (fig. 3) leaves little 

 doubt that the range of natural distribution conforms with this 

 indicated classification. The halepensis, or rootstock, forms are 

 confined to Asia Minor, Turkey, Greece, Italy, southern France, and 

 the northern parts of Africa, the distribution extending eastward 

 through southern Asia to the Himalayas, while farther south in the 

 interior of Africa all the wild forms of Andropogon seem to lack 

 rootstocks and to be more closely related to the true sorghums. 



Rather strong evidence of a specific difference between Johnson 

 grass and the sorghums is the difficulty attending their cross-pollina- 

 tion. It has long been known that Johnson grass crosses very rarely 

 with the sorghums, even though the two species have been intermin- 

 gled in the same fields for the last 30 years. A letter of inquiry was 

 sent in 1912 to each agronomist of the agricultural experiment sta- 

 tions of our Southeastern States where Johnson grass was known to 

 be abundant, asking him if he had ever personally observed an un- 

 doubted hybrid of Johnson grass and sorghum. The replies revealed 

 the astonishing fact that only one of these men so intimately in touch 

 with agricultural conditions in these States was willing to say that 

 he had observed even what he suspected might be a cross between 

 these two crop plants. Since that time a definite search for such 

 hybrids has resulted in the discovery of three or four undoubted 

 natural crosses between sorghum and Johnson grass, one of which is 



3 The serial numbers in parentheses refer to " Literature cited," at the end of this bulletin. 



! The technical name of Sudan grass used in Department Bulletin No. 772, entitled ' ' The Genera of Grasses 

 of the United States," by A. S. Hitchcock, p. 267, is Holcus sorghum sudanensis (Piper) Hitchc, while 

 its allies are known as Holcus sorghum drummondii (Nees) Hitchc, and Holcus sorghum ex:guus (Forsk.) 

 Hitchc. 



