Bark anatomy of Euphorbia tirucalli (Euphorbiaceae): A unique way of bark dilatation on a stem succulent tree

The covering tissue structure is important for leafless stem succulents, influencing their photosynthetic activity. Usually, the epidermis on succulent stems persists for a long time, making the stem photosynthesize as long as possible. Its persistence implies maintaining the continuity of covering tissues during bark dilatation. In most plants, dilatation is performed by developing periderm(s) to replace the epidermis. The ways of bark dilatation with delay of periderm formation remain poorly known. In the present study, we examine the structure of the juvenile and mature bark of succulent pencil tree Euphorbia tirucalli (Euphorbiaceae) to clarify the ways of dilatation of its protective tissues. The bark structure of Euphorbia tirucalli at various developmental stages was examined using light and scanning electron microscopy. The epidermis is found not only on young stems of E. tirucalli but also on the bark of its thick branches and trunk, which are up to about 6 cm in diameter. On the young stems, the stomata are sunken in long vertical grooves. On the older stems, these grooves are stretched out due to dilatation, bringing stomata to the surface. Expansion cracks, i.e. broad vertical epidermal ruptures underlaid by tightly arranged tangential strands of cortical parenchyma, appear between the stomatal zones on dilated bark. The phellogen is initiated in the depth of the cortex beneath the epidermal ruptures long after their formation. The bark dilatation by forming epidermal ruptures with delayed initiation of periderm was found for the first time. Prolonged photosynthesis in the succulent stem is the main functional benefit of such a bark dilatation method. The initiation of periderm in the depth of the cortex has not been reported in stem succulents other than a few Euphorbia species.