Plant Taxonomy 2 PDF

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Dr. Sanaa Abdel Rahman Mostafa Zaghlool, Dr. Fatma Saeed

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plant taxonomy botany gymnosperms plant biology

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This document provides a detailed explanation of plant taxonomy, specifically focusing on gymnosperms. It covers various plant phyla, their characteristics, and reproductive strategies. The content presents information about seed plants, focusing on the exposed nature of their seeds.

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Plant Taxonomy 2 Dr. Sanaa Abdel Rahman Mostafa Zaghlool Dr. Fatma Saeed Agric Botany Department Seed Plants: Gymnosperms ❑ The name gymnosperm is derived from two Greek words: gymnos, meaning naked, and sperma, a seed. ❑ The name refers to the exposed nature of the seeds, whi...

Plant Taxonomy 2 Dr. Sanaa Abdel Rahman Mostafa Zaghlool Dr. Fatma Saeed Agric Botany Department Seed Plants: Gymnosperms ❑ The name gymnosperm is derived from two Greek words: gymnos, meaning naked, and sperma, a seed. ❑ The name refers to the exposed nature of the seeds, which are produced on the surface of sporophylls or similar structures instead of being enclosed within a fruit as they generally are in the flowering plants. A comparison between exposed gymnosperm seeds and enclosed angiosperm seeds. A. Exposed seeds on a woody seed cone of a pine tree. B. A single seed cone scale with two seeds. C. A section through an apple (angiosperm fruit), showing the enclosed seeds. Four phyla of living gymnosperms are recognized ❑ Phylum Pinophyta includes about 575 species of pines, firs, spruces, hemlocks, cedars, redwoods, and other coniferous woody plants. ❑ Fossils of some conifers extend back 290 million years to the late Carboniferous period. ❑ Phylum Ginkgophyta has a single living representative, Ginkgo, which is a tree with fan-shaped leaves. ❑ Phylum Cycadophyta, the superficially palmlike cycads are assigned to it. ❑ Phylum Gnetophyta includes three genera of gnetophytes that have wood with vessels—a structural feature unknown in other gymnosperms. Phylum Pinophyta The Conifers - Pines ❑ The largest genus of conifers, Pinus (pines) has over 100 living species. ❑ They are the dominant trees in the vast coniferous forests of the Northern Hemisphere. ❑ They include the world’s oldest known living organisms, some trees still standing are about 4,600 years old. Structure and Form ❑ Pine leaves are needlelike and are arranged in clusters of two to five leaves each. ❑ Each cluster (fascicle) forms a cylindrical rod of the leaves are held together. ❑ The fascicles are short shoots with restricted growth, a feature of some gymnosperms not found in flowering plants. ❑ Most gymnosperm wood, including that of pines, consists primarily of tracheids and differs from the wood of dicots in having no vessel members or fibers. ❑ Conifer wood, because of the absence of thick-walled cells, is said to be soft, while the wood of broadleaf trees is described as hard. ❑ In many conifers, the annual rings of the xylem are often fairly wide as a result of a comparatively rapid A portion of a cross section of a pine stem, showing annual rings. growth rate during the growing season). Reproduction Pines produce two kinds of spores. ❑ Pollen cones (male strobili) consist of papery or membranous scales arranged in a spiral or in whorls around an axis. ❑ The pollen cones usually develop toward the tips of the lower branches in clusters of up to 50 or more and are mostly less than 4 centimeters long. A cluster of pine pollen cones. ❑ Microsporangia develop in pairs toward the bases of the scales. ❑ Each of the microsporocytes in the microsporangia undergoes meiosis, producing four haploid microspores. ❑ These then develop into pollen grains; each grain consists of four cells and a pair of external air sacs. ❑ The air sacs look something like tiny water wings) and Pollen grains of a pine. Each pollen give the pollen grains added buoyancy that may result in grain has a pair of air sacs that the grains being carried great distances by the wind. provides added buoyancy. ❑ Megaspores are produced in megasporangia located within ovules at the bases of the seed cone scales. ❑ The seed cones (female strobili) are much larger than the pollen cones, becoming as much as 60 centimeters long in sugar pines and weighing as much as 2.3 kilograms in Coulter pines. ❑ When mature, they have woody scales, with inconspicuous bracts between them, arranged in a spiral around an axis. ❑ They are mostly produced on the A B upper branches of the same tree on Seed cones of pines. A. Immature cones shortly which the pollen cones appear. after being produced. B. A mature cone. ❑ The ovules occur in pairs toward the base of each scale of the immature seed cones and are larger and more complex than the microsporangia of pollen cones. ❑ Each ovule has within it a megasporangium containing the nucellus and a single megasporocyte. ❑ Megasporocyte is surrounded and enclosed by a thick, layered integument. ❑ The integument has a tubular channel or pore called a micropyle. ❑ One of the integument layers later becomes the seed coat of the seed. A longitudinal section through a pine ovule ❑ A single megasporocyte within the megasporangium of each ovule undergoes meiosis, producing a row of four relatively large megaspores. ❑ Three of the megaspores soon degenerate. Over a period of months, the remaining one slowly develops into a female gametophyte. ❑ The nucellus is used as the food source for the growing gametophyte. ❑ As gametophyte development nears completion, two to six archegonia differentiate at the end facing the micropyle. Each archegonium contains a single large egg. ❑ Seed cones are at first usually reddish or purplish in color and commonly take two seasons to mature into the green and finally the brownish woody structures with which we are all familiar. ❑ During the first spring, the immature cone scales spread apart, and pollen grains carried by the wind sift down between the scales. ❑ There they catch in sticky drops of fluid (pollen drops) oozing out of the micropyles. ❑ As the fluid evaporates, the pollen is drawn down through the micropyle to the top of the nucellus. ❑ After pollination, the scales grow together and close, protecting the developing ovule. ❑ Meanwhile, the pollen grain (immature male gametophyte) produces a pollen tube that slowly grows and digests its way through the nucellus to the area where the archegonia develop. ❑ While the pollen tube is growing, two of the original four cells in the pollen grain enter it. ❑ One of these, called the generative cell, divides and forms two more cells, called the sterile cell and the spermatogenous cell. ❑ The spermatogenous cell divides again, producing two male gametes, or sperms. ❑ The sperms have no flagella and are confined to the pollen tube until just before fertilization occurs. ❑ The germinated pollen grain, with its pollen tube and two sperms, constitutes the mature male gametophyte. ❑ About 15 months after pollination, the tip of the pollen tube arrives at an archegonium, unites with it. ❑ One sperm unites with the egg, forming a zygote. The other sperm and remaining cells of the pollen grain degenerate. ❑ The sperms of other pollen grains present may unite with the eggs of other archegonia, and each zygote begins to develop into an embryo that is nourished by the female gametophyte. ❑ At a later stage, an embryo may divide in such a way as to produce the equivalent of identical twins or quadruplets in animals. ❑ Embryos develop at the ends of long suspensor cells. ❑ Normally, however, only one embryo completes development. ❑ While this development is occurring, one of the layers of the integument hardens, becoming a seed coat. ❑ A thin membranous layer of the cone scale becomes a “wing” on each seed. The wing may aid in the seed’s dispersal. Life cycle of a pine Phylum Cycadophyta—The Cycads ❑ Cycads have the appearance of a cross between a tree fern and a palm but are related to neither. ❑ These slow-growing plants have unbranched trunks that grow more than 15 meters tall in a few species and have a crown of large, pinnately divided leaves. ❑ Cycad life cycles are similar to those of conifers, except that pollination of cycads is sometimes brought about by beetles instead of wind. ❑ Cycads are dioecious, and both the pollen strobili and the seed strobili of some species are A B huge (e.g., more than a meter long with a weight A. A male cycad with a strobilus. B. A female of over 220 kilograms. cycad with a strobilus.

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