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

## TL;DR

This study reveals a unique bark expansion method in the succulent Euphorbia tirucalli, where stomata move to the surface through stretching grooves and cracks, allowing prolonged photosynthesis.

## Contribution

The first documentation of bark dilatation in Euphorbia tirucalli through epidermal ruptures with delayed periderm formation.

## Key findings

- Stomata on Euphorbia tirucalli stems are initially in deep grooves but move to the surface as the stem expands.
- Bark expansion occurs via epidermal ruptures underlaid by cortical parenchyma strands, with periderm forming later.
- Phellogen initiation occurs deep in the cortex, a rare trait among stem succulents.

## Abstract

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.

The stem of Euphorbia tirucalli , a succulent plant, undergoes remarkable structural changes after leaf shedding. Stomata, essential for gas exchange, are positioned within deep stomatal grooves that facilitate the movement of carbon dioxide into the underlying tissue (cortical parenchyma). As the stem expands, these stomatal grooves stretch, eventually bringing the stomata to the surface. Unlike most plants, where bark expansion is driven by periderm growth, the epidermis in E. tirucalli  remains intact on thick branches and trunks but expands through the formation of expansion cracks rather than uniform stretching. These ruptures are underlaid by densely packed tangential strands of cortical tissue beneath them. Interestingly, the periderm develops only after these cracks form, a process not previously documented in other plants. Additionally, the formation of the phellogen (a layer responsible for producing protective bark tissue) occurs unusually deep in the cortex, an uncommon trait among Euphorbia species and other stem succulents. The study also reclassifies sclerenchyma cells in the bark as fibre-sclereids rather than traditional fibres, refining our understanding of this plant’s anatomy.