Rhizoids are not absorptive. Without a well-defined system to transport water throughout their bodies, where would you guess that these types of plants are found? And if you had never seen a bryophyte before, but you knew it had no vascular system, would you suspect the plant to be tall or have a low stature? Bryophytes and mosses are found in moist environments, and they are typically low to the ground, because they lack stems or a vascular system to transport water.
There are several key innovations that our buddies the bryophytes had to undergo to make the evolutionary leap from an aquatic to terrestrial environment. Two structural changes allowed these plants to survive in a dry, terrestrial atmosphere. First, bryophytes developed cuticles, different from the layer of skin at the base of our fingernails and toenails. A plant cuticle is a waxy layer that covers the plant that keeps water in and keeps the plant from drying out.
Second, bryophytes developed stomata, which are pores in the cuticle that allow gas exchange. Sure, plants release oxygen, but they also need to take in carbon dioxide for respiration, so pores are critical for gas exchange to take place. The last key innovation for bryophytes is a reproductive adaptation. Remember, that in bryophytes the gametophyte generation is dominant. Being that this plant is a gamete producing plant, they do it with style.
Bryophytes developed gametangia or specialized gamete-forming structures. There are two types: the male, sperm producing structure antheridium and the female, egg producing structure archegonium. Bryophytes, for all the strides they made to adapt to living on land, have a carryover from their aquatic ancestry — they still need water for reproduction. Sperm released from the antheridium will swim to the archegonium to fertilize the egg.
The developing zygote actually grows up out of the gametophyte into the spore-producing generation sporophyte. The diploid sporophyte is totally dependent upon the gametophyte for survival. The key innovation for this next group, and for all plants from this point forward in our survey of plant diversity, is vascular tissue: xylem to transport water and minerals, and phloem to transport sugar.
Vascular plants have true leaves, stems, and roots. Vascular plants also have a special substance called lignin which is a compound in the cell walls of plants that gives them additional strength and stability.
The seedless vascular plants include club mosses, whisk ferns, horsetails, and ferns. Some of the plants in this group still need water for fertilization. Unlike the bryophytes, the sporophyte generation is dominant.
The gamete-producing plant, or gametophyte generation is free-living but very small. The key innovation for the remaining plants is the development of seeds. This may sound simple, but the development of seeds was a major adaptation in the evolution of plants.
Seeds are hearty, and most importantly, they can endure dry conditions. The adaptation of the seed meant that plants were free from their dependency on water for reproduction, and consequently they could colonize drier environments. Basically this means the production of two distinct types of spore producing structures, and therefore two distinct types of spores: microspores and megaspores.
The microspores develop into pollen, or the male gametophyte. The megaspore develops into the egg, or the female gametophyte. The egg and sperm fuse to form a zygote that develops into an embryo, which is protected inside several layers and wrapped in a protective coat. The whole package is the seed! We group the seed plants into two major groups: gymnosperms and angiosperms.
The exciting aspect of these two groups is that the sporophyte is dominant and the gametophyte is so reduced that it is dependent upon the sporophyte for survival. The sperm and egg develop within the sporophyte, and the female gametophyte is retained within the tissues of the sporophyte. Most importantly, plants produce the oxygen that we breathe. Animals also eat plants in order to obtain energy. Organisms in the Plant Kingdom differ from other kingdoms in that they all contain chlorophyll which is necessary to perform photosynthesis, they are stationary and cannot move from one spot to another, and their cell walls contain cellulose.
Cellulose is a material that gives plants their crispness. They have needle-like shaped leaves with a thick waxy cuticle Mature naked seeds occur at bases of female cones some months after pollination Class Cycadales Cycadeles resemble palms superficially. They have long compound leaves which are clustered at the apex of a thick ussualy short and unbranched stems.
They have cones which are borne at the apex of the trunk among leaves. Class Ginkgoales Members of this class are very rare. Examples include Ginkgo biloba species in china 2. Characteristics They are flower bearing and are usually bisexual. Seeds are enclosed in an ovary which develops into a fruit. Xylem has tracheids and vessels while phloem has companion cells. They exhibit double fertilization.
This group is divided into Two Classes a Class Dicotyledonae These are plant whose embryo of seeds has two cotyledons. Their leaves are broad and have networks of veins Cross section of stems reveals vascular bundles arranged in rings They have taproot system Centrally placed star-shaped xylem with phloem alternating with arms of the xylem Their flowers have floral parts in five or fours Examples are herbs, shrubs, and trees.
The herbs include plants with all stems such as beans, cabbages, tomatoes and black jack while the shrubs include plants with fairly thick stems such as coffee, tea and cocoa b Class Monocotyledonae Their seeds have an embryo with one cotyledon Relatively narrow leaves with parallel veins The cross section of the stem reveals scattered vascular veins No vascular cambium hence no secondary growth They bear floral parts in threes Examples include Maize, grass, wheat, sorghum, sugarcane, coconuts, bananas and sisal.
Climate change. Cooperation with Anin. Land plants include four main groups: bryophytes, pteridophytes, gymnosperms and angiosperms. Mosses are the most familiar bryophytes, and most of them lack true vascular tissues, so they are sometimes referred to as nonvascular plants.
The first plants to live on land also lacked vascular tissue, like mosses and liverworts. Ferns, which are pteridophytes, do not make seeds, so they are sometimes called "seedless plants. Conifers, pines and ginkgos are gymnosperms, which means "naked seeds" in Greek. Gymnosperms make seeds, but their seeds are not protected by fruit. The last of the four groups is the angiosperms; they are also called the "flowering plants.
Angiosperms are distinguished by the fact that they produce flowers, and they also produce fruits to protect their seeds. Different classification systems list anywhere from ten to 14 divisions of the plant kingdom.
Plant taxonomy, like life itself, is constantly evolving.
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