- crown
The part of a tree that consists of its branches and foliage extending from the trunk or main stems.
In the mature Celtis australis, horizontal boughs spread out in all directions from the crown, creating layered shade and structure.
- crustose lichen
A type of lichen that forms a thin, crust-like layer tightly adhered to a surface, often rock or bark, and cannot be removed without damaging the substrate.
Crustose lichens are found growing on the bark of Celtis australis in Lisbon, forming a patchy mosaic on older trunks and branches.
- cuticle
A protective, waxy layer covering the epidermis of plant surfaces, minimising water loss and shielding against environmental stress.
The cuticle on sun-exposed Celtis australis leaves is thick and parchment-like, helping the tree retain moisture during the dry Lisbon summer.
- diffusion
The passive movement of molecules from an area of higher concentration to one of lower concentration, facilitating gas exchange in plants.
Carbon dioxide enters the leaves of Celtis australis through the stomata by diffusion, a process that is minimal in dry conditions.
- drupe
A fleshy fruit in which an outer layer encloses a hard shell containing a single seed.
Celtis australis produces small drupes in autumn, which provide food for pigeons that nest in its branches.
- epidermis
The outermost layer of cells covering the leaves and young parts of a plant, providing protection and regulating interactions with the environment.
The upper epidermis of Celtis australis leaves feels like fine-grained sandpaper, while the lower epidermis is covered with fine hairs.
- foliose lichen
A type of lichen with a leaf-like structure that is loosely attached to the surface it grows on, often with visible lobes and a distinct upper and lower surface.
Foliose lichens, with their ruffled forms, are among the symbionts growing on Celtis australis, especially in shaded and moist parts of the crown.
- glabrous
Smooth and lacking hairs or bristles, often describing the surface of leaves or stems.
The upper side of Celtis australis leaves is glabrous, in contrast to the tomentose underside.
- lichen
A symbiotic organism composed of a fungus and a photosynthetic partner, usually algae or cyanobacteria, forming crusty, leafy, or branching growths on surfaces.
Crustose and foliose lichens grow on the bark of Celtis australis in Lisbon, forming part of the tree’s urban symbiotic network.
- moss
Small, non-vascular plants that form dense green clumps or mats, often in moist or shaded environments, reproducing via spores.
Moss grow on the shaded trunks of Celtis australis, absorbing moisture where there is a humid microclimate.
- petiole
The stalk that connects a leaf blade to the stem, supporting the leaf and enabling nutrient transport between leaf and plant.
The leaves of Celtis australis emerge asymmetrically from the petiole, which attaches them to the boughs and positions them to catch light.
- photosynthesis
The process by which green plants convert light energy, water, and carbon dioxide into oxygen and sugars essential for growth.
Although photosynthesis is reduced in the dry heat of midsummer, Celtis australis retains its leaves in preparation for more favorable conditions in autumn.
- senescence
The process of biological aging in plants, involving the gradual deterioration of cells, tissues, or organs, often leading to leaf yellowing and eventual death.
In autumn, the leaves of Celtis australis show senescence, turning yellow before falling, which allows the tree to conserve resources during winter.
- stomata
Microscopic pores found on leaf surfaces that regulate gas exchange and water loss by opening and closing in response to environmental conditions.
In the high temperatures of July, the stomata of Celtis australis remain closed, conserving water but halting growth.
- symbiotic
Describing a close and often long-term biological interaction between two different species, which may benefit one or both parties.
Celtis australis has symbiotic relationships with pigeons, which shelter in its branches, and with lichens that grow on its bark.
- tomentose
Covered with densely matted, soft hairs, often serving to reduce water loss or deter herbivory.
The tomentose underside of Celtis australis leaves helps slow down water loss and may offer some protection from leaf-eating insects.
- transpiration
The movement of water from the roots to aerial parts of a plant, followed by its evaporation from leaf surfaces into the atmosphere.
To survive Lisbon’s summer heat, Celtis australis reduces transpiration by limiting water loss through its leaf surfaces.