Vol. 7, 1921 
BOTANY: HARRIS AND SINNOTT 
35 
THE VASCULAR ANATOMY OF NORMAL AND VARIANT 
SEEDLINGS OF PHASEOLUS VULGARIS 
By J. Arthur Harris and Edmund W. Sinnott 
Station for Experimental Evolution, Carnegie Institution of Washington 
Communicated by C. B. Davenport, November 29, 1920 
The investigations here summarized comprise a comparative and bio- 
metric study of the gross vascular anatomy of normal and variant seed- 
lings of Phaseolus vulgaris. 
Three morphological types have been considered, (a) the normal or 
dimerous seedling with two cotyledons and two primordial leaves, (b) 
the trimerous seedling with three cotyledons and three primordial leaves, 
and (c) the hemitrimerous seedling in which there are three cotyledons and 
two primordial leaves. 
In normal seedlings, the vascular system of the root is typically tetrarch 
(with four protoxylem poles), and gives rise in the base of the hypocotyl 
to four pairs of double bundles which soon form a circle of eight bundles 
which continue to the cotyledonary node. At this point there is a com- 
plex vascular anastomosis. From it two strands are given off to each 
cotyledon. The remainder of the vascular tissue is reorganized into six 
strands, each of which typically soon divides into two, the twelve bundles 
thus formed comprising the vascular system of the epicotyl. 
The trimerous seedlings typically possess six root poles instead of four, 
twelve bundles in the hypocotyl instead of eight, and nine primary epico- 
tyledonary bundles instead of six. The nine primary epicotyledonary 
bundles do not all divide, however, so that the number of bundles in the 
central region of the epicotyl is variable ranging in general from fourteen 
to eighteen. 
In both classes of seedlings, but more frequently in the normal type, 
additional or intercalary bundles appear in the hypocotyl, either de novo 
or as a result of division of the primary strands. 
Four main groups of problems as to the vascular topography of these 
seedling types have been considered biometrically : First, the number of 
bundles at different levels in the seedling; second, the variability in bundle 
number; third, the differentiation in internal structure of seedlings which 
are externally dimerous, trimerous and hemitrimerous; and fourth, the 
interrelationship of bundle number in different regions of the seedling. 
The following table of constants 1 summarizes the facts for number 
and variability of vascular bundles in various regions of the seedling and 
in the three types of seedlings. 2 
The constants in this table, and the frequency distributions from which 
the constants were computed, lead to the following conclusions. 
