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STOMATA In plants, a stoma is a tiny pore in the surface of a leaf that is used for gas exchange. ... The term “stoma” comes from the Greek word for “mouth.” Groups of more than one stoma are often called “stomata” rather than “stomas,” because “stomata” is the Greek way of pluralizing “stoma.” Stomate, also called stoma, plural stomata or stomas, any of the microscopic openings or pores in the epidermis of leaves and young stems. Stomata are generally more numerous on the underside of leaves. They provide for the exchange of gases between the outside air and the branched system of interconnecting air canals within the leaf. A stomate opens and closes in response to the internal pressure of two sausage-shaped guard cells that surround it. The inner wall of a guard cell is thicker than the outer wall. When the guard cell is filled with water and it becomes turgid, the outer wall balloons outward, drawing the inner wall with it and causing the stomate to enlarge. Guard cells work to control excessive water loss, closing on hot, dry, or windy days and opening when conditions are more favourable for gas exchange. For most plants, dawn triggers a sudden increase in stomatal opening, reaching a maximum near noon, which is followed by a decline because of water loss. Recovery and reopening are then followed by another decline as darkness approaches. In plants that photosynthesize with the CAM carbon fixation pathway, such as bromeliads and members of the family Crassulaceae, stomata are opened at night to reduce water loss from evapotranspiration. The concentration of carbon dioxide in the air is another regulator of stomatal opening in many plants. When carbon dioxide levels fall below normal (about 0.03 percent), the guard cells become turgid and the stomata enlarge.

BULB Bulb, in botany, a modified stem that is the resting stage of certain seed plants, particularly perennial monocotyledons. A bulb consists of a relatively large, usually globe-shaped, underground bud with membraneous or fleshy overlapping leaves arising from a short stem. A bulb’s fleshy leaves—which in some species are actually expanded leaf bases—function as food reserves that enable a plant to lie dormant when water is unavailable (during winter or drought) and resume its active growth when favourable conditions again prevail. red onion; yellow onion Red and yellow onions (Allium cepa).

There are two main types of bulbs. One type, typified by the onion, has a thin papery covering protecting its fleshy leaves. The other type, the scaly bulb, as seen in true lilies, has naked storage leaves, unprotected by any papery covering, that make the bulb appear to consist of a series of angular scales. Bulbs can vary in size from insignificant pea-sized structures to those of large crinum lilies (Crinum species), the individual bulbs of which may weigh more than 7 kg (15 pounds). bulb Plants sprouting out of the ground from bulbs.

Bulbs enable many common garden ornamentals, such as the narcissus, tulip, and hyacinth, to produce their flowers rapidly, almost precociously, in early spring when growing conditions are favourable. Other bulb-producing plants, such as the lilies, flower in the summer, while a few, such as the meadow saffron, bloom in the fall. Bulb-producing species are especially abundant in the lily (Liliaceae) and amaryllis (Amaryllidaceae) families. A few bulb-producing species are of economic importance to humans because of the taste and nutritive value of their fleshy leaves; included among such species are the onion and its relatives the shallot, garlic, and leek. Other bulbs contain poisonous compounds—such as the red squill (Drimia), the bulbs of which are the source of a highly effective rat poison. A hyacinth (Hyancinthus) bulb, an underground stem that produces aerial foliage. The foliage dies back to the bulb, which houses a maturing flower bud. The bulb is common in herbaceous perennials that become dormant in response to a seasonal change in climate or water availability.

In horticulture the term bulb is incorrectly applied to a number of botanical structures that have a similar food-storing function. Among these are the solid corms of the crocus and gladiolus and the elongated rhizome of some irises.