Fig. 10.3-9. Transverse section of corn stem (Zea mays). Stems of corn, like those of many monocots, have a very fibrous construction. Here, the vascular bundles are enclosed by bundle sheath fibers and the outermost cortex consists of several layers of fibers. Most fiber tissue is compact, with few or no intercellular spaces, which inhibits the diffusion of gases. Notice that the band of fibers in the outer cortex is interrupted near the stoma: just below the stoma is a mass of blue-stained parenchyma cells with intercellular spaces. It is not quite open enough to call it an aerenchyma, but it does permit carbon dioxide to pass through the epidermis and be absorbed by the chlorenchyma in the cortex.

            How is this controlled? Does the initiation of guard mother cells prevent the formation of fibers? Or does the fibrous cortex leave gaps in itself, and those gaps stimulate the differentiation of guard mother cells?