Primary vascular bundle - Fig

…The arrangement of xylem and phloem varies among plant groups and organs. There are parallel bundles (stem and leaves of angiosperms and gymnosperms) with xylem on the inside and phloem on the outside of a single vascular bundle, compound parallel bundles (Cucurbitaceae, etc.) with phloem on the inside of the xylem, encircling bundles (excircling bundles are common in the stems and leaves of ferns), and radial bundles (roots of vascular plants) with xylem and phloem arranged alternately to form a ring. Primary vascular bundles differentiate from the procambium that differentiates near the meristems of the shoot apex, root tip, and leaf margin. Differentiation does not proceed simultaneously, but varies depending on the part of the vascular bundle. The bundle that differentiates first at the apical part where elongation is active is called protoxylem or protophloem, and the bundles that differentiate later are called metaxylem or metaphloem. …

*Some of the terminology that mentions "primary vascular bundles" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…木部と師部の配列は植物群や器官によってさまざまであり，1本の維管束で木部が内側に師部が外側にある並立維管束(被子植物と裸子植物の茎と葉)，木部の内側にも師部がある複並立(両立)維管束(ウリ科など)，一方が他方に囲まれる包囲維管束(外師包囲維管束はシダ植物の茎と葉にふつう)，木部と師部が交互にならび環状をなす放射維管束(維管束植物の根)などがある。一次維管束は茎頂･根端および葉縁の各分裂組織の近くで分化した前形成層から分化する。分化は一斉にすすむのではなく維管束の部位によって異なり，伸長生長の盛んな頂端部でまず分化するものを原生木部，原生師部，あとから分化するものを後生木部，後生師部という。…

※「一次維管束」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報