8 BULLETIN 1386, U. S. DEPARTMENT OF AGRICULTURE 
can not be easily measured or described, and because there are fre- 
quently transitional forms which prevent the differences being clear 
cut. Although it is sometimes possible to indicate the variations 
in the feature occurring in each variety, when such is the case, 
wide, sharply marked, and constant differences are more valuable 
for purposes of identification. Characters which may be observed 
without magnification have greater value than those which must be 
magnified, and those which are to be seen in all the plants of the 
variety are of course more dependable than those which occur in only 
a portion of them. Characters which relate to parts of the plant 
which are deciduous are never of much value for distinguishing 
varieties of a cultivated species. 
Natural fluctuation occasioned by external influences or other causes 
often results in an overlapping of characters even when the means are 
far apart, thus making it difficult to distinguish between the varie- 
ties, and when the means are close together the difficulty of dis- 
criminating between them on account of such overlapping is of course 
greater. Although fluctuations are probably due mainly to environ- 
mental influences, it should be remembered that there are other 
causes which may also be effective, as follows: (1) One or more of 
the varieties concerned may contain a number of biotypes; (2) the 
ages of the various stalks and panicles in a field always differ some- 
what; and (3) the mode of expression of some of the factors con- 
cerned is apparently such that they are represented in the progenies 
alternatively, or in a series of forms, or in varying degrees of in- 
tensity. Many morphological features are much modified by 
causes other than germinal, on which account distinctions based on 
actual size are usually less reliable than those based on the size 
character as compared with that of adjacent parts or those based on 
form. It is to be noted that there are marked differences in the 
extent to which various features of the sorgo plants are in this way 
affected by environmental influences, the height of the plant being 
modified the most, the size of the panicle nearly as much, and the 
dimensions of the spikelets apparently the least. Since the length 
of panicle branches arising at various nodes of the axis partly deter- 
mines its form, fluctuations in form are in some cases ascribable to the 
fact that the panicle branches on the upper part of the axis develop 
a little before those on the lower part, soil or climatic conditions 
being somewhat different at the two periods. 
DESCRIPTION OF PLANT 
VEGETATIVE PORTION 
The sorghum plant, whether belonging to the sorgo or the non- 
saccharine group, consists essentially of one or more culms, or stalks, 
having pithy interiors and arising from the root system. Each culm 
is composed of a series of internodes which decrease in diameter 
from the base of the stalk upward. As in other species of the 
Graminea3, a bud and a leaf, the latter consisting of a sheath and 
a blade, originate at each node. The sheaths in this group of 
sorghums for the most part completely cover the internodes above, 
although there are a few varieties in which some of the sheaths 
