Topic 13 - Plant Science

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Biology exsam notes Topic 9.1

Topic 9.1.1 Draw and label plan diagrams to show the distribution of tissues in the stem and leaf of a dicotyledonous plant.

topic 9.1.2 Outline three differences between the structures of dicotyledonous and monocotyledonous plants

MONOCOTS DICOTS

Embryo with single cotyledon Embryo with two cotyledons Pollen with single furrow or pore Pollen with three furrows or pores Flower parts in multiples of three Flower parts in multiples of four or five Major leaf veins parallel Major leaf veins reticulated Stem vacular bundles scattered Stem vascular bundles in a ring Roots are adventitious Roots develop from radicle Secondary growth absent Secondary growth often present

• Number of cotyledons -- The number of cotyledons found in the embryo is the actual basis for distinguishing the two classes of angiosperms, and is the source of the names Monocotyledonae ("one cotyledon") and Dicotyledonae ("two cotyledons"). The cotyledons are the "seed leaves" produced by the embryo. They serve to absorb nutrients packaged in the seed, until the seedling is able to produce its first true leaves and begin photosynthesis. • Pollen structure -- The first angiosperms had pollen with a single furrow or pore through the outer layer (monosulcate). This feature is retained in the monocots, but most dicots are descended from a plant which developed three furrows or pores in its pollen (triporate). • Number of flower parts -- If you count the number of petals, stamens, or other floral parts, you will find that monocot flowers tend to have a number of parts that is divisible by three, usually three or six. Dicot flowers on the other hand, tend to have parts in multiples of four or five (four, five, ten, etc.). This character is not always reliable, however, and is not easy to use in some flowers with reduced or numerous parts. • Leaf veins -- In monocots, there are usually a number of major leaf veins which run parallel the length of the leaf; in dicots, there are usually numerous auxillary veins which reticulate between the major ones. As with the number of floral parts, this character is not always reliable, as there are many monocots with reticulate venation, notably the aroids and Dioscoreales. • Stem vascular arrangement -- Vascular tissue occurs in long strands called vascular bundles. These bundles are arranged within the stem of dicots to form a cylinder, appearing as a ring of spots when you cut across the stem. In monocots, these bundles appear scattered through the stem, with more of the bundles located toward the stem periphery than in the center. This arrangement is unique to monocots and some of their closest relatives among the dicots. • Root development -- In most dicots (and in most seed plants) the root develops from the lower end of the embryo, from a region known as the radicle. The radicle gives rise to an apical meristem which continues to produce root tissue for much of the plant's life. By contrast, the radicle aborts in monocots, and new roots arise adventitiously from nodes in the stem. These roots may be called prop roots when they are clustered near the bottom of the stem. • Secondary growth -- Most seed plants increase their diameter through secondary growth, producing wood and bark. Monocots (and some dicots) have lost this ability, and so do not produce wood. Some monocots can produce a substitute however, as in the palms and agaves.

Topic 9.1.3 Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues. • Xylem: Bring water to replace losses due to transpiration, and inorganic minerals from the soil. • Phloem: Transports products of photosynthesis out of the leaf. • Stoma: A pore that allows CO2 for photosynthesis to diffuse in and O2 to diffuse out. • Guard Cells: this pair of cells can open or close the stoma and so control the amount of transpiration. • Upper Epidermis: a continuous layer of cells covered by a thick waxy cuticle. It prevents water loss from the upper surface even when heated by sunlight. • Lower Epidermis: is in a cooler position and has a thinner waxy cuticle. • Spongy mesophyll: consists of loosely packed rounded cells with few chloroplasts. This tissue provides the main gas exchange surface so must be near the stomata in the lower epidermis. • Palisade mesophyll: consists of densely packed cylindrical cells with many chloroplasts. This is the main photosynthetic tissue and is positions near the upper surface where the light intensity is highest Topic 9.1.4Identify modifications of roots, stems and leaves for different functions: bulbs, stem tubers, storage roots and tendrils.

Topic 9.1.5. State that dicotlydenous plants have apical and lateral meristems.

Topic 9.1.6 Compare growth due to apical and lateral meristems in dicotyledonous plants.

Topic 9.1.7 Explain the role of auxin in phototropism as an example of the control of plant growth