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)* and not Andropogon 
halepensis, under which name it was obtained from Africa.‘ 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 Kurope 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) Hitche., while 
its allies are known as Holcus sorghum drummondii (Nees) Hitche., and Holcus sorghum exiguus (ForsK.) 
Hitche. 
