The Growth And Development Of Frond And Pinna In Five Species Of Cycads

The growth and development of frond and pinna in five species of cycads was studied by growth observation and microscopy of tissue development. This study observed morphological change , growth rate, tissue differentiation of pinna ,and stomatal ontogeny and stomatal density of epidermis. It could provide clue for cultivation of cycad and provide information for taxonomy, conservation, evolution and identification of their fossil.These cycad species had different growth patterns. The longitudinal ptyxis of the whole leaf was erect. The horizontal ptyxis was involute in Cycas, conduplinate in Encephalartos. The individual pinna was circinate in Cycas, flat in Dioon and Encephalartos.Tlie pinna in each side of a frond overlapped each other succubously in Dioon and Encephalartos.The growth and development of frond in these observed species showed clear positive correlation between the elongation's time of frond, pinna and their mature size.The development of vascular bundle was similar in these species: both primary xylem and primary phloem were exarch, and the secondary xylem was near the phloem. The studied species hadsimilar leaflets structure initially and became distinct during their development. Cycas debaoensis had idioblasts both in mesophyll tissue and assimilation tissue of midrib. Cycas miquelii had druse crystals in the mature mesophyll tissue. The three Cycas species were different in the structure of hypodermal cells and palisade tissue. Dioon spinulosum and Encephalartos gratus were different in the arrangement of mucilage canal and vascular bundle(the former is corresponding, but the latter is alternate), distribution pattern of idioblasts and drusecrystals. Idioblasts occured in different tissue in different developmental stages.The somatal ontogeny was haplocheilic as the guard cells(GC) and subsidiary cells(SC) developed from different mother cells in the observed species. The three Cycas species showed no notable differences in the development of epidermis and stomata. When the epidermis became mature, rounded or oval pits developed on cell periphery, near to the anticlinal walls . The guard cells started to sunk one or two cell layers with the development. Two stomata apparatus occurred side by side forming stomata clusters and arised from two adjacent CMC. Premature stomata apparatus were often present near the midrib. Hair bases were composed of one or two cells. They were similar initially and became distinct with development.Hair bases in Dioon spinulosum and Encephalartos gratus occurred only in leaf margin. Their epidermis and stomata were similar in early developmental stages. Mature epidermis cnosisted of two category of epidermal cells with thick or thin walls respectively. No pits were observed in the surface wall. The GMC in Dioon spinulosum was sunken below the surface in epidermis but in Encephalartos gratus it did not occur. The mature guard cells were deeply sunken to form a three or four cell layer deep pit in the former but two or three cell layer deep pit in the latter. The periclinal division of SC brought out the GC sunk.Stomatal density peaked differently in different species. But ,in one species stomatal density varied dramatically from the base to the tip in the initial developmental stage and became evenly distributed in the end.The results from the study of morphology, ontogeny of epidermis and stomata and stomatal density showed that the leaflets in the observed species matured acropetaly.