Fig. 16.3-1a and b. Transverse section of cotton stem (Gossypium). As the vascular cambium produces secondary xylem and secondary phloem, the pre-existing phloem from earlier years is pushed outward and stretched circumferentially. It undergoes cell division and enlargement, a process called dilatation growth when it occurs in secondary phloem or cortex (note the spelling – this is “dilatation,” not “dilation”) and it produces dilatation tissue. If the parenchyma cells of the rays dilatate, that is called proliferative tissue. If parenchyma cells in the axial masses of the phloem divide and expand, that is called expansion tissue. These two micrographs show proliferative tissue: at the vascular cambium, the ray is uniseriate – one cell wide – as is the xylem ray. But further out, the phloem ray is three cells wide (upper arrow in high magnification micrograph): the extra cells are due to divisions of the ray cells themselves. See pages 349 and 350 in Plant Anatomy for more details.