Abstract
Plant structures that enclose trapped air are morphologically and taxonomically diverse. They range from pubescence (trichomes) on various parts of plants to flowers, inflorescences, stems, culms (above-ground jointed stems of grasses), petioles, peduncles, scapes, fruits, bracts, leaves, galls, algal pneumatocysts, moss sporophytes, lichen podetia, and fungal fruiting bodies. Despite being familiar, such structures have not been studied systematically until recently when their complex thermodynamic functionality as microgreenhouses has been recognized. We propose the term “heliocaminiform” (Greco-Latin origin for “sun-room”) provides an umbrella term that describes form and function. Almost all the hollow structures we have examined have elevated internal temperatures of several degrees C above the surrounding air in sunshine, but those are abolished under cloud or at night. The potential importance for the additional heat is presumed to be in growth, maturation, reproduction, sexual function, and overall fitness of the plants. There seem to be no experimental studies on those effects even though they may help explain aspects of plants’ responses to climate change and to phenological mismatches with symbionts (mutualists and herbivores) as ecologically co-dependent partners. Our review and observations opens a remarkably new and hitherto surprisingly neglected avenue in botany which we hope others will explore.
Graphical Abstract
Introduction and etymology
Hollow structures have been little studied by botanists even though widespread and diverse. They occur in Angiospermae (e.g., in stems, culms, flowers, inflorescences, fruits, petioles, peduncles, spadices, and galls with or without additional pubescence), moss sporophytes, lichens (podetia and thalli), Phaeophyta (pneumatocysts), and fungi (stipes and caps). Several recent papers have pointed out the possible importance of such hollow structures in the lives of some plants and their symbionts (Kevan et al. 2018; van der Kooi et al. 2019; Coates and Kevan 2021) with their contributing significantly to heat budgets, growth, maturation, and reproduction through the microgreenhouse effect. A few studies propose that photosynthetic gas exchange (especially CO2) is ameliorated and adaptive within hollow structures (e.g., some subalpine plants (Billings and Godfrey (1967) (Table 1, row 34), wheat (Triticum aestivalis (Poacaea)) culms (Bornemisza-Pauspertl et al. 1984), the inflated stems of Eriogonum inflatum (Polygonaceae) (Osmond et al. 1987), and algal pneumatocysts (Table 1, row 39). With such a broad array of structures across so many phylogenetic lines, it is appropriate that formal recognition be given to the structures and how they may function. We propose a term and its definition as follows: Heliocaminiform structures are small, hollow parts of plants (sensu lato) that function as microgreenhouses. The term derives from the Greco-Latin word “heliocaminus” in reference to the solar heated room in the palace of Roman emperor Hadrian (b. 24 January 76 – d. 10 July 138) who reigned from 117 to 138. It is also recorded that Tiberius (Roman emperor Tiberius Caesar Augustus b. 16 November 42 BC—d. 16 March AD 37) commissioned the building of greenhouses for cucumber growing earlier. Readers interested in that early history of greenhouses may wish to consult Pliny the Younger (1900) (1st Century AD; Firth (1900) translator) and Justinian (1904–1909)(5th Century AD; Monro and Buckland (1904-1909) translators)).
