The major problem facing a plant living on land is how can it allow carbon dioxide from the air to pass freely into its leaves without at the same time losing excessive quantities of water by evaporation to the surrounding atmosphere which is usually relatively dry? In the chrysanthemum the outer surfaces of the leaf are protected by a thin waterproof layer (cuticle), and the main pathway by which water leaves the leaf is through the stomata--small pores in the upper and lower leaf surface (see diagram 1.4).
The stomata open during the day to admit carbon dioxide to the large air spaces in the leaf and close at night, when photosynthesis ceases, so as to minimise this loss of water.
In cross-section (see diagram 1.4) can be seen that the chrysanthemum leaf is very thin. This ensures that carbon dioxide entering through the stomata has only a short distance to travel before it can pass into a photosynthesising cell in the middle of the leaf. These cells are packed with chloroplasts containing the green pigment chlorophyll. On the other hand, the thinness of the leaf means that some light passes right through and is not absorbed by the chloroplast pigments.
Light absorption is greatest when the leaves present a broad, fiat, dark-green surface to the incoming light. This can be achieved by avoiding wilting at any stage and by supplying the correct nutrients, especially magnesium which is an important constituent of chlorophyll, at the right concentrations. The spiral arrangement of leaves around the stem also helps to form a broad column of light-absorbing tissue. Indeed, if the object is to get maximum photosynthesis per unit area of ground, the plants should be closely spaced so that all the incoming light is absorbed by foliage and none reaches the soil.
Growers of exhibition blooms, however, are usually more concerned with getting maximum production from each plant. For this purpose the plants should be well spaced so that they cast relatively little shade on one another, and the growing area should be sited where the plants will not be shaded by any neighbouring trees, buildings or other objects. Having arranged for the plants to receive the maximum of light it is also essential to keep their leaf surfaces free of dusts, powders and spray residues, otherwise some of the light will be unable to reach the chlorophyll-containing cells in the middle of the leaf and will be wasted.
A further feature of nutrient uptake is that the accumulation is selective and some nutrients can be absorbed in preference to others, even though they may be present in lower concentration in the growing medium. The best policy, however, is to ensure that the nutrients are present in the medium in the appropriate proportions.
In a well-aerated medium, nutrient uptake proceeds normally and the root hairs have a higher concentration of nutrients than in the growing medium. As a result, water passes across the semi-permeable membranes of the root hairs and enters the root by osmosis. It then passes across to the transport tissues at the centre of the root which are continuous with those in the stem and the veins of the leaf.
The arrangement of transport tissues in the stem is shown in the diagram. Water and nutrients move together in the xylem; a group of large, dead cells with thick walls which form a series of continuous hollow tubes that run from the roots to the leaves. Organic materials are carried in the small, living, thin-walled cells of the phloem. Each phloem cell is a separate unit, but they have many connections with their neighbours.
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Last updated on 26 June, 2002