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AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
and the smallest containing only one or two lignified xylem cells and a small 
patch of phloem. Some are even more reduced, consisting of a phloem 
patch alone. Any strand in which at least one well lignified xylem element 
could be made out was counted as a bundle. Some of the bundles are 
partially double in character, this condition being due either to partial 
fusion or to incipient division. Whenever such a strand was surrounded 
by one bundle sheath it was counted as one bundle; when the separation 
was so great that the bundle sheath itself showed signs of division, the 
strand was counted as two. 
The seedlings were harvested at a stage when the vascular tissues of 
the first epicotyledonary internode were not completely lignified, and the 
number of bundles counted was therefore possibly less than the number 
which would finally be developed there. 
None of these possible sources of error is believed to be great enough to 
afifect the conclusions appreciably. 
The Structure of the Seedling 
In order to provide a sound basis for the understanding and inter- 
pretation of our later work, it is necessary to present a brief descriptive 
account of the structure of the seedlings. 
The Normal (Dimerous) Seedling 
The morphology of the seedling of Phaseolus has received the attention 
of several investigators, notably DodeP and Compton.^ Like most of the 
large seedlings of the Leguminosae it is normally tetrarch in fundamental 
plan; that is, there are four groups of protoxylem elements in the root. At 
a very early stage there is associated with each of these a group of metaxylem 
cells. It is these groups of metaxylem elements, throughout the whole 
seedling, which in the present paper are counted as "bundles," even though 
(as is sometimes the case) they are not associated with protoxylem clusters. 
At the stage when these seedlings were harvested, cambial activity had 
hardly begun to show itself, so that these primary bundles remained distinct 
and easy to identify. 
The condition in the upper part of the root of a normal seedling is shown 
in figure i. The four bundles, two in the cotyledonary plane and two in the 
intercotyledonary plane, are more or less V-shaped (with the protoxylem 
group in an exarch position at the apex of the V) and tend to extend laterally. 
They surround a large pith. In passing up into the base of the hypocotyl, 
each of these bundles divides into two (fig. 2), and typical stem structure, 
^ Dodel. A. Der Ubergang des Dicotyledonen-stengels in die Pfahl-wurzel. Pringsh. 
Jahrb. 8: 149-193. 1872. 
^ Compton, R. H. An investigation of the seedling structure in the Leguminosae. 
Jour. Linn. Soc. 41 : 7-122. 1912. 
