Root biomechanics in Rhizophora mangle: anatomy, morphology and ecology of mangrove's flying buttresses.

BACKGROUND AND AIMS: Rhizophora species of mangroves have a conspicuous system of stilt-like roots (rhizophores) that grow from the main stem and resemble flying buttresses. As such, the development of rhizophores can be predicted to be important for the effective transmission of dynamic loads from the top of the tree to the ground, especially where the substrate is unstable, as is often the case in the habitats where Rhizophora species typically grow. This study tests the hypothesis that rhizophore architecture in R. mangle co-varies with their proximity to the main stem, and with stem size and crown position. METHODS: The allometry and wood mechanical properties of R. mangle (red mangrove) trees growing in a mangrove basin forest within a coastal lagoon in Mexico were compared with those of coexisting, non-buttressed mangrove trees of Avicennia germinans. The anatomy of rhizophores was related to mechanical stress due to crown orientation (static load) and to prevailing winds (dynamic load) at the study site. KEY RESULTS: Rhizophores buttressed between 10 and 33 % of tree height. There were significant and direct scaling relationships between the number, height and length of rhizophores vs. basal area, tree height and crown area. Wood mechanical resistance was significantly higher in the buttressed R. mangle (modulus of elasticity, MOE = 18·1 ± 2 GPa) than in A. germinans (MOE = 12·1 ± 0·5 GPa). Slenderness ratios (total height/stem diameter) were higher in R. mangle, but there were no interspecies differences in critical buckling height. When in proximity to the main stem, rhizophores had a lower length/height ratio, higher eccentricity and higher xylem/bark and pith proportions. However, there were no directional trends with regard to prevailing winds or tree leaning. CONCLUSIONS: In comparison with A. germinans, a tree species with wide girth and flare at the base, R. mangle supports a thinner stem of higher mechanical resistance that is stabilized by rhizophores resembling flying buttresses. This provides a unique strategy to increase tree slenderness and height in the typically unstable substrate on which the trees grow, at a site that is subject to frequent storms.

Seed germination and seedling morphology of Smilax polyantha (Smilacaceae) / Germinação e morfologia de plântulas de Smilax polyantha (Smilacaceae)

Brazilians have been using the underground organs of Smilax species in alternative medicine since the 19th century because of their anti-rheumatic qualities. However, even nowadays, these species are explored only by extractivism. Studies on seed germination and development of these organs could be useful to preserve these plants. After germination, seedling development of Smilax polyantha was analyzed to understand underground stem formation. Furthermore, to analyze the ontogenesis of the underground system, seedlings aged from one to twelve months were sectioned. One of the most striking features of this species is the presence of two stem branching systems. The plumule gives rise to the first stem branching system with negative geotropism. Its first underground axillary bud sprouted into the other caulinar axis with positive geotropism. The horizontal growth and the subsequent thickening of this underground organ depended on the development of axillary buds from basal nodes of the previous branches. The cotyledonary bud did not play a role in the underground formation, as previously described in the literature for this genus, but the buds of the basal cataphylls built the second stem branching system. In this study we discuss the terminology and suggest calling this second stem branching system a rhizophore.

As espécies de Smilax L. são utilizadas na medicina popular brasileira desde o século 19 devido às propriedades anti-reumáticas atribuídas aos órgãos subterrâneos de todas as espécies. No entanto, ainda hoje, essas espécies são exploradas apenas por extrativismo. Estudos sobre a germinação e o desenvolvimento dos órgãos subterrâneos podem ser úteis para preservar essas plantas. Após a germinação, o desenvolvimento de plântulas de Smilax polyantha foi analisado para compreender a formação do sistema subterrâneo. Para a análise da ontogênese do sistema subterrâneo foram seccionadas plantas em diferentes estágios de desenvolvimento entre um e doze meses. Uma das características mais marcantes desta espécie é a presença de dois sistemas de ramificação caulinar. A plúmula dá origem ao primeiro sistema caulinar de ramificações com geotropismo negativo. As gemas axilares subterrâneas desse primeiro eixo caulinar originam o segundo eixo caulinar com geotropismo positivo. O crescimento horizontal e o espessamento do órgão subterrâneo dependem do desenvolvimento de gemas axilares de nós basais dos ramos anteriores. A gema cotiledonar não participa da formação do caule subterrâneo, como descrito anteriormente na literatura para este gênero, mas as gemas axilares basais dos ramos caulinares aéreos originam o segundo eixo de ramificação caulinar subterrâneo. Neste estudo, após ser discutida a terminologia mais adequada, sugere-se chamar o caule subterrâneo dessas espécies de rizóforo.