ABSTRACT
Stem succulence evolved independently in many plant lineages as an adaptation to arid environments. One of the most interesting cases is the convergence between Cactaceae and Euphorbia, which have anatomical adaptations mostly to increase photosynthetic capability and water storage. Our goal was to describe the shoot development in two succulent species of Euphorbia using light microscopy coupled with high-resolution X-ray-computed tomography. Collateral cortical bundles were observed associated with the stem ribs in both species. The analysis of vasculature demonstrated that these bundles are, in fact, leaf traces that run axially along a portion of the internode. That structural pattern is due to an ontogenetic alteration. During shoot development, the leaf-bases remain adnate to the stem near the SAM, forming an axial component. When the internode elongates, the leaf bundles stretch as cortical bundles. The meristematic activity associated with the bundles forms the stem ribs, as leaf veins near the node, and induce rib formation along the entire internode even in the portion where the leaf traces join the stele. In addition, heterochronic shifts are also involved in the evolution of the shoot system in these Euphorbia, being related to early deciduous reduced leaves and the transference of the main photosynthetic function to the stem. This study demonstrates for the first time the influence of leaf developmental shifts and stem rib formation in Euphorbia and sheds new light on the evolution of stem succulence.
ABSTRACT
The growth in thickness of monocotyledon stems can be either primary, or primary and secondary. Most of the authors consider this thickening as a result of the PTM (Primary Thickening Meristem) and the STM (Secondary Thickening Meristem) activity. There are differences in the interpretation of which meristem would be responsible for primary thickening. In Cordyline fruticosa the procambium forms two types of vascular bundles: collateral leaf traces (with proto and metaxylem and proto and metaphloem), and concentric cauline bundles (with metaxylem and metaphloem). The procambium also forms the pericycle, the outermost layer of the vascular cylinder consisting of smaller and less intensely colored cells that are divided irregularly to form new vascular bundles. The pericycle continues the procambial activity, but only produces concentric cauline bundles. It was possible to conclude that the pericycle is responsible for the primary thickening of this species. Further away from the apex, the pericyclic cells undergo periclinal divisions and produce a meristematic layer: the secondary thickening meristem. The analysis of serial sections shows that the pericycle and STM are continuous in this species, and it is clear that the STM originates in the pericycle.The endodermis is acknowledged only as the innermost layer of the cortex.
O crescimento em espessura do caule de monocotiledônea pode ser primário, ou primário e secundário. A maioria dos autores consideram o espessamento resultante do MEP (Meristema de Espessamento Primário) e do MES (Meristema de Espessamento Secundário). Há divergências de qual seria o meristema responsável pelo espessamento primário. Em Cordyline fruticosa o procâmbio forma feixes vasculares de dois tipos: traços foliares colaterais (com proto e metaxilema e proto e metafloema), e feixes caulinares concêntricos (com metaxilema e metafloema). O procâmbio também forma o periciclo, a camada mais externa do cilindro vascular, constituída por células menores e menos coradas que se dividem irregularmente, formando novos feixes vasculares. O periciclo dá continuidade à atividade procambial, originando somente feixes concêntricos. Concluiu-se ser o periciclo responsável pelo espessamento primário desta espécie. Mais distante do ápice as células pericíclicas passam a sofrer divisões periclinais originando o Meristema de Espessamento Secundário. A análise dos cortes seriados mostra que o periciclo e o MES são contínuos nesta espécie, ficando claro que o periciclo origina oMES. A endoderme é reconhecida, apenas, como a camada mais interna do córtex.