Morpho-anatomy of species in Tribe Pachycereeae seedlings. Until when are they seedlings? [Morfo-anatomía de plántulas en especies de pachycereeae: ¿Hasta cuándo son plántulas?]

Morpho-anatomy of species in Tribe Pachycereeae seedlings is described and compared with the aim to find differences between the initial growth and establishment stages and to determine the point in time of the change from seedling to young plant. The morpho-anatomical traits were observed from sampling more than 500 seedlings of six species belonging to five genera of this tribe. Seedlings were collected weekly the first three-months after germination and afterwards they were collected monthly to complete 24-months of growth. Statistical differences (P < 0.05) were observed for seedling height, root length, and stem width and height between the species at different ages. A delay in stem and root development was found in Escontria chiotilla and Myrtillocactus geometrizans, species with the smallest seeds. Moreover, the studied species share dermal, fundamental, and vascular tissue traits, however the protein bodies distinguish Pachycereus grandis and the prismatic crystals in epidermis-hypodermis characterize species of Neobuxbaumia. At the onset of vascular cambium, only vessel elements and wide band tracheids become differentiated, whereas fibers appeared in some species after they had one-year of age. We concluded that disappearance of cotyledons, differentiation of ribs, establishment of vascular cambium, a greater accumulation wood, and differentiation of medullary bundles allow us to determine the limits between the seedling stage and the young plant stage