How and why does the areole meristem move in Echinocereus (Cactaceae)?
OBJECTIVE
In Cactaceae, the areole is the organ that forms the leaves, spines and buds. Apparently, the genus Echinocereus develops enclosed buds that break through the epidermis of the stem adjacent to the areole; this trait most likely represents a synapomorphy of Echinocereus. The development of the areole is investigated here in order to understand the anatomical modifications that lead to internal bud development and to supplement anatomical knowledge of plants that do not behave according to classical shoot theory.


METHODS
The external morphology of the areole was documented and the anatomy was studied using tissue clearing, scanning electron microscopy and light microscopy for 50 species that represent the recognized clades and sections of the traditional classification of the genus, including Morangaya pensilis (Echinocereus pensilis).


RESULTS
In Echinocereus, the areole is sealed by the periderm, and the areole meristem is moved and enclosed by the differential growth of the epidermis and surrounding cortex. The enclosed areole meristem is differentiated in a vegetative or floral bud, which develops internally and breaks through the epidermis of the stem. In Morangaya pensilis, the areole is not sealed by the periderm and the areole meristem is not enclosed.


CONCLUSIONS
The enclosed areole meristem and internal bud development are understood to be an adaptation to protect the meristem and the bud from low temperatures. The anatomical evidence supports the hypothesis that the enclosed bud represents one synapomorphy for Echinocereus and also supports the exclusion of Morangaya from Echinocereus.
Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal, 04510, Mexico and Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal 04510, Mexico.
Daniel
D
Sánchez
Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal, 04510, Mexico and Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal 04510, Mexico.
Dalia
D
Grego-Valencia
Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal, 04510, Mexico and Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal 04510, Mexico tterrazas@ib.unam.mx.
Teresa
T
Terrazas
Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal, 04510, Mexico and Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, México Distrito Federal 04510, Mexico.
Salvador
S
Arias
D000222:Adaptation, Physiological; D029421:Cactaceae / Q000033:anatomy & histology / Q000145:classification / Q000254:growth & development* / Q000648:ultrastructure; D018519:Meristem / Q000033:anatomy & histology / Q000254:growth & development* / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D009024:Morphogenesis; D013045:Species Specificity
Areole meristem; Cactaceae; Echinocereus; enclosed bud; enclosed meristem; erumpent bud; periderm.; sealed areole