How a cactus stem retains its shape during drought

Water changes stem dimensions

A cactus stem changes volume with its water reserves. Parenchyma, which fills a substantial part of its interior, remains sensitive to changes in turgor. As hydration decreases, tissues can shorten and narrow. The epidermis, ribs and vascular system must accommodate these deformations while maintaining the organ's continuity. The anatomy of a water-storing plant therefore includes both elements that can deform and elements that stabilise its shape. [Vázquez-Sánchez et al. 2017]

Two small Mexican cacti illustrate distinct aspects of this organisation. Aztekium ritteri has a repeatedly folded surface whose closure shelters the more delicate parts of the epidermis. In Geohintonia mexicana, attention shifts to the stem interior. The type of wood changes there during development. The first example concerns the geometry of the boundary between the plant and the atmosphere, and the second concerns tissue support and water conduction. [Porembski 1996][Vázquez-Sánchez et al. 2017]

A large surface hidden in its own shade

In Porembski's 1996 analysis, A. ritteri had ribs divided by transverse grooves that sometimes branched into even finer crevices. The total surface area of this sculpted stem was about ten times that of a corresponding smooth sphere. However, about 80% remained sheltered by neighbouring folds. The area directly exposed to radiation accounted for approximately one fifth of the total. [Porembski 1996]

A cross-section reveals the significance of this structure. Protruding portions had a thick cuticle, strongly thickened hypodermal cell walls and numerous calcium oxalate crystals in the epidermis. Deeper surfaces consisted of more delicate cells with a thinner cuticle. Almost the entire stomatal population was concentrated there. The plant thus separated the areas most exposed to external factors from most of the surface involved in gas exchange. [Porembski 1996]

Grooves closing as turgor falls

A dehydrated stem loses some of its internal tension. The walls of neighbouring grooves move closer together and conceal the stomata deeper inside. The most robust parts remain on the outside, while softer parts withdraw into the system of crevices. Turgor influences the accessibility of the entire gas-exchange zone here. This regulation arises from stem geometry and operates alongside the action of the guard cells themselves. [Porembski 1996]

Porembski presented this arrangement as a functional adaptation to periodic water shortage. His study covered morphology, anatomical sections and scanning microscopy of plants cultivated in Bonn. It did not quantify the reduction in transpiration following groove closure. Reduced water loss is an interpretation of the observed surface organisation. The hypothetical contribution of crystals to radiation reflection and of roughness to heat exchange with the air was considered separately. [Porembski 1996]

Tracheids with wide wall bands

In many Cacteae, wood contains wide-band tracheids, referred to in the literature as WBTs. They are short and broad, and their wall thickenings form rings or spirals. Thinner regions of the primary wall remain between the bands of secondary wall. This heterogeneous construction combines local reinforcement with the ability to deform. Vessels running through such tissue may also retain a spiral arrangement of thickenings. [Vázquez-Sánchez et al. 2017]

In 2017, Vázquez-Sánchez and colleagues distinguished wood without fibres, wood with few fibres, wood with fibres grouped in patches and dimorphic wood. They found the first of these types in A. ritteri and A. hintonii. With a small proportion of strongly lignified elements, the vascular tissue can shorten vertically more readily along with the surrounding parenchyma. The authors related this property to the stem's response to dehydration. [Vázquez-Sánchez et al. 2017]

Geohintonia changes the type of wood it forms

Dimorphic wood forms in two developmental phases. Wide tracheids dominate initially, then the cambium begins producing tissue containing fibres. Fibres have more strongly lignified walls and increase stiffness. Vessel-element wall structure also changes. In the older phase, they form more extensive secondary walls with pits through which water is exchanged between neighbouring elements. [Vázquez-Sánchez et al. 2017]

This wood type was found in Geohintonia mexicana. It is particularly interesting because this small plant has a globose stem. In many other Cacteae studied, dimorphic wood was associated with greater height. Geohintonia shows that the timing of fibre production depends on the plant's developmental and evolutionary history in a way more complex than a simple relationship with size. [Vázquez-Sánchez et al. 2017]

Greater wood stiffness limits its ability to compress vertically, but stem deformation remains possible through the other tissues and its geometry. G. mexicana has been described as having a parenchymatous cortex and pith surrounded by a thick-walled hypodermis. Wood, the layer beneath the epidermis and the ribs together form the support system. Considering one of these elements alone omits some of the forces acting in the hydrated organ. [Vázquez-Sánchez et al. 2017]

Several combinations of the same elements

The 2017 study covered 135 Cacteae taxa. The authors compared seven morphological and anatomical traits and then reconstructed their changes on a phylogenetic tree. The anatomical material included additional samples, at least three per taxon. Mainly basal and middle stem portions were analysed. This sample allowed comparison of different combinations of wood, hypodermis, cortex and pith across a broad group of cacti. [Vázquez-Sánchez et al. 2017]

Similar arrangements arose repeatedly in separate lineages. Supporting tissues could reinforce the wood, cortex or pith. Ribs and tubercles altered the spatial pattern of deformation. The authors interpreted these combinations as ways of maintaining growth form during prolonged drought. This was a reconstruction of the evolution of anatomical traits, rather than a comparative experiment subjecting all studied taxa to equal dehydration. [Vázquez-Sánchez et al. 2017]

Stem shape reflects tissue condition

In A. ritteri, changes in hydration are reflected in the opening of the grooves. In Geohintonia, the capacity for deformation also depends on which portion of wood the plant has already produced. In both examples, surface appearance remains related to internal structure. The same decrease in volume may be distributed differently in an organ with soft wood and one with a developed fibrous phase. [Porembski 1996][Vázquez-Sánchez et al. 2017]

Understanding these relationships requires combining anatomical description with measurements. Microscopy establishes cell distribution and structure. Controlled dehydration would allow measurement of volume changes, gas exchange and the force needed to deform the stem at successive hydration states. The cited studies provide detailed anatomical foundations and hypotheses that can be tested through precisely such experiments. [Porembski 1996][Vázquez-Sánchez et al. 2017]