[Reprinted from the Proceedings of the NATIONAL ACADBMY OF SCIBNCBS, 

 Vol. 7, No. 1, pp. 35-41, January, 1921.] 



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, (6) 

 the trimerous seedling with three cotyledons and three primordial leaves, 

 and (e] 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. 



