![]() SHOOT‐MERISTEMLESS ( STM) is expressed throughout the SAM except at sites of incipient leaf primordia at its base STM appears either to inhibit cells from differentiating or to promote cell proliferation within the centre of the SAM ( Endrizzi et al., 1996). The roles of several genes are becoming understood ( Nakajima and Benfey, 2002). The size of the SAM results from a balance between production of new stem cells within the meristem and loss of cells to production of leaf primordia, nodes and internodes ( Howell, 1998 Lyndon, 1998). Leaf primordium initiation at the base of the shoot apical meristem (SAM) establishes phyllotaxy, while spatial relationships of pith, vascular bundles, cortex and epidermis are laid out in the subapical meristem. The shoot apex is the site in which a shoot’s morphology is established. These giant SAMs should make it possible to more accurately map gene expression patterns relative to SAM zonation and LP sites. thaliana, and genes such as PERIANTHIA might establish much more extensive fields of inhibition around LP. Conclusions Presumably, genes such as SHOOT‐MERISTEMLESS, WUSCHEL and CLAVATA must control much larger volumes of SAM tissue in cacti than they do in A.There is almost no correlation between SAM diameter and LP size, but SAM diameter is strongly correlated with shoot diameter, with shoots being about 189♵ times wider than SAMs. Species with high phyllotaxy have smaller LP, although the correlation is weak. ![]() ![]() Leaf primordium diameter ranges from 44 to 402 µm, each encompassing many more cells than do LP of other plants. Phyllotaxy ranges from distichous to having 56 rows of leaves and is not restricted to Fibonacci numbers. thaliana and having a volume 45 thousand times larger. Key Results Cactus SAM diameter varies from 93 to 2565 µm, the latter being 36 times wider than SAMs of A.Apices from 183 species of cacti were fixed, microtomed and studied by light microscopy. This study examined how SAM size affects leaf primordium (LP) size, phyllotaxy and shoot diameter. Some cacti have SAMs much larger than those of A. Background and Aims Shoot apical meristems (SAMs) in most seed plants are quite uniform in size and zonation, and molecular genetic studies of Arabidopsis and other model plants are revealing details of SAM morphogenesis.Moreover, some plants break the rules for instance, fuchsias have four petals. 19th Century botanists and mathematicians described the geometry of plants and figured out why Fibonacci numbers arise from it - but they couldn't explain the geometry itself. The scales of pine-cones are similar, and so are the seeds in the head of a ripe sunflower. One family winds anticlockwise and contains 8 spirals the other winds clockwise and contains 13. For instance, the hexagonal segments of pineapples form two interlocking families of spirals. His numbers became known as Fibonacci numbers, and they turn up all over the plant kingdom. Leonardo was the son of Bonaccio, and he later acquired the nickname Fibonacci. Each number is obtained by adding the previous two. His somewhat unrealistic rabbit population increased according to a fascinating sequence of numbers: 1 1 2 3 5 8 13 21 34 55 89 144 233. ![]() These numbers have been familiar to mathematicians since 1202, when the Italian mathematician Leonardo of Pisa posed a problem about rabbits. Marigolds have 13 petals, asters have 21, daisies have 34, 55, or 89, and sunflowers have 55, 89, or 144. Since ancient times, people have noticed strange numerical patterns in plants. A Saguaro Cactus outside Cave Creek Arizona
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