维管形成层是植物中纵向贯穿根和茎的一层组织，位于木质部和韧皮部之间，维管形成层一般由多层细胞所组成，严格说来，其中只有一层原始细胞，可以不断地向内外分裂而增生细胞，不断增生的子细胞分化为木质部和韧皮部以及射线的细胞，使得植物的根和茎能不断地生长加粗。属于侧生分生组织。

维管形成层一般存在于裸子植物和被子植物的双子叶植物中，但在单子叶植物中基本没有维管形成层，所以单子叶植物生长不能不断地加粗，绝大部分为草本植物，只有棕榈科具有乔木状，也是依靠维管束发展，不能有充分的次生生长能力。

The vascular cambium (plural cambia) is a plant tissue located between the xylem and the phloem in the stem and root of a vascular plant, and is the source of both the secondary xylem growth (inwards, towards the pith [material at the center of plant, often dead and/or deteriorated, that is composed of parenchyma tissue]) and the secondary phloem growth (outwards [to the bark, rough or smooth, of the plant]). It is a cylinder of unspecialized meristem cells that divide to give new cells which then specialize to form secondary vascular tissues.