Growth and Development of Horticultural Crops (1+1)

Patterns of Growth and Development-Some Features of Plant Growth

• Growth in plants is restricted to certain zones, recently produced by cell division in a meristem. It is easy to confuse growth (as defined above as an increase in size) with cell division in meristems. Cell division alone does not cause increased size, but the cellular products of division also increase in volume and cause growth. Root and shoot tips (apices) are meristematic in nature.



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• Other meristematic tissues are found in the vascular cambium and just above the nodes of monocots or at the bases of grass leaves. The root and shoot apical meristems are formed during embryo development, while the seed develops and are called primary meristems. The vascular cambium and the meristmatic zones of monocot nodes and grass leaves are indistinguishable until after germination; they are secondary meristems.
• Some plant structures are determinate; it grows to certain size and then stops, eventually undergoing senescence and death. Leaves, flowers, and fruits are good examples of determinate structures. On the other hand, the vegetative stem and root are indeterminate structures. They grow by meristems that continuously replenish themselves, remaining live. A bristlecone pine that has been growing for 4,000 years could probably yield a cutting that would form roots at its base, producing another tree that might live for another 4,000 years. At the end of the time, another cutting might be taken and so on, potentially forever; that is, plants can be cloned from individual parts. Some fruit trees have been propagated from stem sections for centuries.
• Although indeterminate meristems can be killed, it is potentially immortal. But death is the ultimate fate of determinate structures. When an indeterminate, vegetative meristems becomes reproductive (that is, begins to form a flower), it becomes determinate.
• Although there are borderline cases, entire plants are in a sense either determinate or indeterminate. We use different terms, however; monocarpic species flower only once and then die; polycarpic species after flowering, return to a vegetative mode of growth, and flower at least once more before drying. Most monocarpic species are annuals (live only one year), but there are variations in them. Many annuals germinate from seeds in the spring, grow during the summer and autumn, and die before winter, perpetuating themselves only as seeds. Spring wheat and rye are commercial annuals that are planted in the spring, but seeds of winter wheat or rye germinate in the fall, over winter as seedlings beneath the snow, and flower the next spring.
• Typical biennials, such as beet (Beta vulgaris), carrot (Daucus carota), and henbane (Hyoscyamus niger) germinate in the spring and spend the first season as a vegetative rosette of leaves that dies back in late fall. Such a plant overwinters as a root with its shoot reduced to a compressed apical mersitem surrounded by some remaining protective dead leaves (meristem plus leaves is called a perenniating bud). During the second summer, the apical meristems form stem cells that elongate bolt into a flowering stalk.
• The century plant (Agaves americana) may exist for a decade or more before flowering and once attain flowering, it may die. Though a monocarpic species, it would be called a perennial because it lives for more than two growing seasons. Bamboos (Bambusa and other genera), which may live more than half a century before flowering, die after flowering, is an excellent example of the extreme monocarpic growth habit. Polycarpic plants, perennials by definition, do not convert all their vegetative meristems to determinate reproductive ones. Woody perennials (shrubs and trees) may use only some of their axillary buds for the formation of flowers, keeping the terminal buds vegetative; alternatively, terminal buds may flower while axillary buds remain vegetative. Sometimes, a single meristem forms only one flower, as in a tulip, whereas single grass or Asteraceae meristem forms an inflorescence or head of flowers (for example, sunflower, bottle brush, Callistemon sp.)