Tabitha Norton Essays - Botany, Biology, Plant Morphology

Tabitha Norton Essays - Botany, Biology, Plant Morphology Tabitha Norton Bio 102-03 Selaginella is the sole genus of vascular plants in the family Selaginellaceae, the spike mosses. in all parts of the world, particularly in the tropics such as Puerto Rico and the Virgin Islands but are also foun din as more arctic habitats in areas of Greenland. They have scalelike leaves, either spirally arranged or in ranks of four, on stems and branches. The spore-bearing leaves are similar to ordinary leaves but are clustered in spikes, or strobili. The spike mosses reproduce with spores. They have distinct male and female spores known as microspores and megaspores, respectively. Spore are produced on leaves in enclosures called sporangia. The spores of Selaginella species are both pollinated and dispersed by wind. Pinus is the sole genus in the subfamily Pinoideae and is made up of conifers. There are about 120 species and are native to northern temperate areas but are found throughout the world. The genus is composed of trees or shrubs, aromatic, evergreen; they are usually conic when young, often rounded or flat-topped with age. Pines have two types of branches, long shoots and short shoots, and three types of leaves, primordial, scale, and adult. Both long and short shoots develop in the axils of the deciduous scale leaves. The needlelike photosynthetic adult leaves are borne in fascicles (bundles) of two to at the tip of each short shoot; they remain on the tree 2 to 17 years. Pollen-bearing male cones are covered with many fertile scales, each of which bears two pollen sacs. Ovule-bearing female cones, borne on the same tree, have several spirally arranged modified leaves, each of which is located below a scale with two ovules . In spring or early summer the pollen sacs release poll en through longitudinal slits; each grain has two air bladders for wind dispersal. The scales on the female cones open to receive the pollen and then close; actual fertilization takes place late the following spring. After fertilization, the woody female cone develops over a two- to three-year period. Ginkgo biloba includes a group of gymnosperms composed of the family Ginkgoaceae. The species is native to China. A Ginkgo tree is pyramidal in shape and has a bark which is deeply furrowed on older trees and has a corky texture. Ginkgo tree leaves are fan-shaped and are have short but thickened shoots. The leathery leaves are up to 8 cm long and can be broad. Most leaves are divided into two lobes by a central notch. Microsporangia and female ovules are borne on separate trees. Pollen grains are carried to the female trees by the wind. Megasporangiate trees bear paired ovules, which, when fertilized, develop into yellowish seeds consisting of a large inner nutlike portion surrounded by a fleshy outer covering. Magnolia are native to North and South America, the Himalayas, and East Asia. They have evergreen or deciduous, alternate smooth-margined leaves. The flowers, usually cuplike and fragrant, are located at the branch tips and have three sepals, six to 12 petals arranged in two to four series, and many spirally arranged stamens. The numerous simple ovaries in the centre later form a conelike fruit. The seeds hang by slender threads. The gynoecium is the innermost whorl of pistils in the flower and is typically surrounded by thestamens, collectively called the androecium. The gynoecium, the female portion of the flower, produces megaspores, each of which develops into a female gametophyte which then produces egg cells. Antirrhinum -There are about 20 species native to western North America and the western Mediterranean region. The flowers are tubular, bilaterally symmetrical, and usually large with a closed, liplike mouth that excludes most insects but can be forced open by strong bees, the main pollinators. Snapdragons bloom only once a year and grow from very small seeds. It generally takes about three weeks for the seeds to germinate.