Lesson 7

• The embryonic plant is kept in a state of suspended development called

  dormancy
• which ends when germination takes place.

  dormancy
• s a first step in breaking dormancy, the embryo emits a hormone called

  gibberellin
• In monocots, the gibberellin triggers the production of digestive enzymes by the aleurone; in dicots, the digestive enzymes are produced by the cotyledons. These enzymes then proceed to break down the stored food in the endosperm or cotyledons. For example, amylase breaks down starch to form maltose, which is then cleaved by maltase to yield glucose; and other enzymes attack stored proteins, fats, and oils, mobilizing them for the seedling's use.
• Microscopic examination of the root tip will disclose a cap consisting partly of dead or moribund cells at its apex, whose function is largely to protect the tender tissues from abrasion by soil particles, and in addition to serve as a source of growth hormones.
• actual living tissue, however, is undergoing vigorous mitosis. For that reason, this part of the root tip is known as the zone of division. It is an apical meristem, a growing tip of embryonic, differentiating tissue.
• the time they have begun to enlarge, they are left behind in a zone of elongation, which is responsible for most of the lengthwise growth of the root tip. Since new cells are constantly added to this zone by mitosis, however, growth continues indefinitely.
• at the same time, the older portions of the zone of elongation cease to grow and become incorporated into the zone of maturation, where tissue differentiation now begins.
• It is an apical meristem, a growing tip of embryonic, differentiating tissue.

  zone of division
• where tissue differentiation now begins

  zone of maturation.
• which is responsible for most of the lengthwise growth of the root tip.

  zone of elongation
• give rise to differentiated tissues such as phloem and xylem. However, some tissues derived from the primary meristem remain undifferentiated and form the cambium layers of the stem and the similar pericycle layer of the root. These eventually form differentiated tissues themselves, but in such a way as to produce lateral growth and increase in girth.


  primary meristems
• Each leaf originates on the side of the meristem, growing upward as it enlarges and begins to differentiate. Once its growth is well under way, another group of cells next to it and somewhat above it begins to grow outward and upward.
• age and die in all plants, and in some cases the plant senses as a whole.

  Leaves
• As xylem ages, for example, it becomes clogged with resins and turns to heartwood.
• As cells produced by the cork cambium age, they die and become converted to cork
• The part of a perennial plant that ages most obviously — and sometimes spectacularly —is the

  leaf
• In deciduous trees, however, all leaves age and die at about the same time in what happens to be basically a water conservation measure.
• is the process by which plants shed one of their parts,

  Abscission
• is an adaptation that specifically permits loss of leaves.

  abscission layer
• The abscission layer also represents a point of weakness, so that in due time, the yellowed leaves are swirled away by the winds of winter.
• If root and shoot (stem) meristems give rise to all plant tissues, a process of differentiation must occur behind the growing tips of the stem and the root.
• In stems, the earliest trace of differentiation occurs just behind the apical zone of active mitosis.
• in the center of the apical bud is a cylinder that develops into the vascular tissues as it matures. Itis appropriately known as the provascular cylinder. On the exterior, potential epidermis called the protoderm develops. Between the two lies a layer called ground meristem or ground tissue, which becomes the cortex and the pith.
• usually does not occur in monocots.

  Secondary growth
• The trunk of a treelike monocot such as bamboo or a palm tree remains roughly the same diameter from base to crown.; in fact, the diameter of a palm trunk can be somewhat smaller at the base than midway between it and the crown. Yet the trunk of a palm seedling is obviously not nearly as great as is that of the mature tree.
• What happens is a bit different than what occurs in dicots. Immediately behind the apical meristem is a primary thickening meristem that is as great in diameter as the trunk. It is derived from the apical meristem, which continuously enlarges to produce it.
• The primary thickening meristem then lays down vascular and other differentiated tissues to form the stem of the palm tree.
• is a fertilized egg that has the potential to give rise to all the diverse cell types of the complete individual.

  zygote
• In most vertebrate zygotes, the cytoplasm contains yolk, which serves as food for the developing embryo.
• The amount and distribution of yolk vary among different animal groups. Yolk is absent from the human zygote.
• is a process wherein the zygote undergoes a rapid mitosis.

  Cleavage
• By about 24 hours after fertilization, the human zygote has completed the first mitotic division and reached the two-cell stage. Each of the cells of the two-cell stage embryo undergoes mitosis, bringing the number of cells to four.
• Repeated divisions continue to increase the number of cells making up the embryo. At about the 16-cell stage, the embryo consists of a tiny cluster of cells called the morula.
• As cleavage takes place, the embryo is pushed along the uterine tube by ciliary action and muscular contraction.
• By the time the embryo reaches the uterus, on about the fifth day of development, it is in the morula stage.
• During this period, its cells arrange themselves into the form of hollow ball called blastocyst (blastula), eventually forms and nutritive membranes (the chorion and placenta) that surround the embryo.
• little cluster of cells, the inner cell mass, projects into the cavity of the blastocyst. These cells give rise to the embryo itself.
• Implantation of embryo in the endometrium (lining) of the uterus begins on the seventh day of embryonic development.
• During this period, enzymes destroy some tiny maternal capillaries in the wall of the uterus.