66 AMERICAN JOURNAL OF BOTANY 



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 

 affect 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 Dodel 3 and Compton. 4 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 &quot;bundles,&quot; 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, 



3 Dodel. A. Der Ubergang des Dicotyledonen-stengels in die Pfahl-wurzel. Pringsh. 

 Jahrb. 8: 149-193. 1872. 



4 Compton, R. H. An investigation of the seedling structure in the Leguminosae. 

 Jour. Linn. Soc. 41: 7-122. 1912. 



