Secondary Growth in Bougainvillea

The anomalous secondary growth was studied in roots and stems of two species of Bougainvillea. The anomalous cambia arise successively in centrifugal order, each originating among the derivatives of the preceding cambium. Each cambium layer functions bidirectionally producing xylem towards the inside of the axis and phloem towards the outside. The sequence of production of vascular cells is the following: (1) conjunctive tissue and xylem fibres towards the inside; (2) phloem towards the outside; (3) additional xylem with vessels towards the inside and additional phloem towards the outside. The new cambia arise outside the oldest phloem cells of a given increment. This phloem may be nonfunctional and crushed at that time. The phloem and the xylem differentiate from radially seriated derivatives produced sequentially by tangential divisions in the cambium. Divisions among the phloem initials and growth readjustments in the differentiating xylem obscure the radial seriation to a moderate extent.