THE PURPOSE of this investigation was to provide a background for a comparative study of the anatomical effects induced by two phloem limited viruses, the curly-top and the aster-yellows viruses, upon the flax plant (Linum usitatissimum L. var. punjab). Since these viruses induce growth abnormalities within the phloem, an ontogenetic study of the vascular system in healthy plants was undertaken. In keeping with the modern approach in studies of vascularization, procambial development was related to the activity of the shoot apex as expressed in the initiation of leaf primordia. Furthermore, the relation between the various leaf arrangements encountered in different flax shoots and the vascular organization of these shoots was explored. The present paper is a report on this latter aspect of vascular organization. Extensive treatises and critiques of theories on leaf arrangement are found in the literature (Church, 1904, 1920; Hirmer, 1922; Plantefol, 1947, 1948; Richards, 1948, 1951; Snow and Snow, 1934; and others). There are also a few studies that attempt to correlate vascular organization and differentiation with leaf arrangement (e.g., Esau, 1943a, b; Priestley and Scott, 1936, 1937; Scott and Priestley, 1925; Smith, 1941; Sterling, 1945). These investigations present evidence that a close relationship exists between phyllotaxis and vascularization. The present study substantiates this relationship by showing that shoots possessing different phyllotaxes differ in vascular organization. MATERIALS AND METHODS.-Material for study consisted of flax plants grown in gallon cans in greenhouse. After the plants were a month old, periodic collections were made of the vegetative shoots. The material was killed and fixed in chromeacetic-formalin combinations. After dehydration, according to the tertiary butvl alcohol method (Johansen, 1940), the material was embedded in 56?58?C. Fisher's tissuemat. Transverse and longitudinal sections were cut 7 or 10, in thickness. Foster's (1934) tannic acid-ferric chloride and safranin method was employed in staining. As an aid in determining the relationship between the arrangement of leaves and the organization of the vascular system, the following method was adopted. Using a microprojector, diagrams of serial transverse sections of the shoot apex were