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Unit 6 - Plants

Blue Flowers

Plant structure and form

  • The basic structures of a plant consist of a root system that is below the ground and a shoot system that is above the ground

  • The five organs of plants include the vegetative components (roots, stems, and leaves) and the reproductive organs (flowers and fruits)

Root System

  • The roots of the root system absorb water and minerals through small root hairs from the substrate on which they are growing.

  • The roots transport the water and minerals to the vascular tissue through the xylem of the plant

Shoot System

  • The shoot system of a plant consists of stems, which provide support, and leaves, the primary organs of photosynthesis. The terminal bud of the stem is the primary growing tip, located at the apex of the stem. This demonstrates apical dominance, a phenomenon in which the plant stem grows more strongly than side stems by pooling resources to promote elongation. Leaves primarily consist of vascular tissue embedded in ground tissue. They are enclosed by epidermal tissue

Types of plant cells:

  • Parenchyma cells

    • thin, flexible cell walls and perform many of the metabolic functions of the plant

    • In the leaf, parenchyma cells are the site of photosynthesis, whereas in stems they aid in the support and storage

  • Collenchyma cells

    • their cell walls have a varying thicknesses that provides structural support to younger areas of the plant, e.g. new shoots and leaves

  • Sclerenchyma cells

    • primarily provide support. They have thick cell walls and are rigid due to the cellulose in their walls

    • Water-conducting cells

    • in the xylem aid in water transport


Plant Reproduction

Plants reproduce through a process called alternation of generations.  It's named this way because their life cycle alternates between a haploid organism and a diploid organism.  In more primitive plants such as mosses (bryophytes), the haploid phase involves a multicellular haploid being, which we don't see in animal life cycles!

Angiosperm life cycle & double fertilization

  1. After a pollen grain (containing the microspores) is deposited on the stigma, it germinates and grows through the style to reach the ovule

  2. A pollen tube cell extends down through the style to reach the embryo sac

  3. The sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid (3n) cell that develops into the endosperm, which becomes the zygote's nutrient source.   Together, these two fertilization events in angiosperms are known as double fertilization (very important to know)

  4. After fertilization, the fertilized ovule develops into a seed, and the tissues of the ovary become the fruit, usually enveloping the seed.

  5. Embryonic development is suspended for a while, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis

Fruit market

Plant Hormones

Plant hormones help coordinate growth, development, and responses to stimuli.  Quick review: a hormone is a signaling molecule produced in small quantities in one part of the body and is then transported to other regions where it can bind to specific receptors and trigger a response​​

Main plant hormones to know:


  • synthesized in shoot apical meristems and young leaves; either synthesized or transported to developing seeds and fruit

  • stimulates stem elongation, promotes the formation of roots, regulates fruit development, enhances apical dominance, functions in phototropism and gravitropism, promotes vascular differentiation, slows leaf abscission


  • synthesized in roots; transported to other organs

  • regulates cell division of shoots and roots, modifies apical dominance, promotes bud growth, promotes nutrient movement, stimulates seed germination, slows leaf senescence


  • produced in meristems of apical buds and roots, young leaves, and developing seeds

  • stimulates stem elongation, stimulates pollen development, causes the growth of the pollen tube, causes fruit growth and seed development, regulates sex determination, aids in the transition from juvenile to adult phase


  • produced by the majority of the plant's parts.  For example, bananas ripen really fast and can even cause nearby fruits to ripen because they produce a lot of ethylene

  • promotes fruit ripening and leaf abscission, inhibits stem elongation, promotes lateral expansion and horizontal growth, enhances senescence rate, promotes root formation, promotes some flowering

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