Fig. 16.3-2a and b. Transverse section of Dutchman’s pipe (also called birthwort; Aristolochia). This is a young stem with only a small amount of secondary phloem (barely visible in the low magnification view), but you can see that the cortex has thick bands of fibers (arrows). The formation of secondary xylem and phloem has pushed the cortex and its fibers outward, and rather than rupturing, dilatation tissue has formed. There were parenchyma cells among the fibers, and they have divided and enlarged in the region marked by the double-headed arrow (shown in the high magnification view). Because this is dilatation by isolated parenchyma cells in the cortex, it is expansion tissue. The ray itself is also dilatating with proliferative tissue.
Notice that the alignment of the ray, the expansion tissue and the lenticel: the lenticel permits oxygen to diffuse into the stem, and the expansion tissue has created an aerenchymatous path through the fiber band (an intact fiber band would hinder oxygen diffusion). The dilatation of the ray will also create more intercellular spaces that will aid oxygen movement.