Succulence is more than just an expansion of stems and leaves as water stores — "plants with middle-age spread" as a revered botanist once described them to me. A storeroom for water is of no use if it lacks a door to prevent the contents from escaping. The external features that distinguish succulents from other plants are discussed in the next chapter. Internally, the succulent tissue has a distinctive look (1.3, 2.5), with large, thin-walled cells and conspicuous air spaces, suggestive of a sponge. These large, watery cells are further distinguished by being highly polyploid— that is, containing up to 16 times the normal number of chromosomes (page 62). The sap is slimy and bitter to the taste. So much for the legend of the life-giving cactus, discharging cool, refreshing water to the thirsty traveller! Far from being cool, the inside temperature of a cactus is several degrees above that of the air outside, and the survival of such plants is in part due to the ability of their cells to endure temperatures 15 to 20 degrees C (28 to 36 degrees F) above those that would damage other plants. Another feature of these storage cells is their ability to collapse gracefully following water loss and to expand again without bursting, which would be fatal. An extreme example of succulence is reported3 for the South African genus Muiria where the plant body encloses a mass of large cells like frog spawn, each up to 2mm (V|2in) in diameter a nd able to remain plump in a dry room for up to ten days before collapsing.
Unusual pathways of life The observant Romans are credited with having been the first to notice that certain fleshy herbs such as the stonecrop (Sedum acre) taste more bitter when picked in the morning than in the evening. You can easily repeat the test to your own satisfaction. Herein lies the germ of the discovery that succulents differ fundamentally from other plants in certain of their life processes, notably by accumulating acid during the night and using it up again during the day. To understand how this comes about, let us briefly review, with the aid of the diagram (1.2), the key events whereby a mesophyte manufactures its food from air, water and salts in solution in the presence of light and chlorophyll "leaf green". The process, known as photosynthesis, takes place in the green surface tissues of leaves (or stems) during the hours of sunlight. The chlorophyll is contained in lens-shaped bodies, chloroplasts, scattered throughout the cell sap. Water and salts in solution enter the plant through the roots and are conveyed upwards to the photosynthesizing tissues. Air enters through special valves or pores called stomata (2.11), which control the flow by opening or closing, rather like the automatic sliding doors of an airport. The nitrogen and oxygen of the air are not directly usable in gas form by plant life; only the carbon dioxide, a mere 0.03 percent of the total, is extracted and used.
The daily cycle of a mesophyte is summarized in 1.2. The stomata open by day and close at night. Light energy from the sun combines water and carbon dioxide in the presence of chlorophyll to form sugar, a complex organic compound classed as a carbohydrate. A by-product of this reaction is oxygen, which is released to the air. By night there is no sun, hence no photosynthesis.
Chemical energy, as distinct from light energy, is also needed for maintaining the vital processes of plants, and this comes from a process called respiration, which reverses photosynthesis, breaking down rather than building up carbohydrate. Respiration goes on all the time, but
Right [ 1.1): High light intensity and evaporation are problems lor this aloe in South West Alrica The leaves are thick and fleshy; dead, dry leaves form a protective
Below ( 1.2): Daily cycle of photosynthesis in non-succulent and succulent plants
during the day is masked by photosynthesis. Part of the carbohydrate is broken down to give water and carbon dioxide, and oxygen is used up as in the burning of fuel.
The daily cycle in succulent plants is not the same. Here, the stomata remain closed by day and open only at night, when intake of air occurs and its carbon dioxide is extracted and fixed by organic acids produced from the partial breakdown of carbohydrate. During the day these organic acids (malic and isocitric mainly) break down into carbon dioxide and water, which are taken up direct and used in photosynthesis, as in a meso-phyte. This daily cycle was first noted in a member of the Crassulaceae Family in 1813* and hence bears the name Crassu-lacean Acid Metabolism, or CAM for short. Recently, two other deviations from the mesophytic cycle have been given the symbols C3 and C4. because they involve the formation of 3- and 4-carbon carboxylic acids. They are mentioned here because there is a strong correlation between the distribution of CAM and C4 systems and that of succulence throughout the flowering plants. Fig. 1.6 makes this clear. It seems as if succulence could evolve only in company with a change in the "normal" respiratory cycle. Now why should this be? The most likely answer is that it is one of many strategies that enable a plant to minimize water loss. When stomata open, as they must in order to allow gas exchanges to take place, they also allow precious water vapour to escape. In mesophytes this is rarely harmful, because more moisture can be drawn in from the soil. Evaporation is much less in the cool night air. and succulents in general adopt the cycle of closing stomata by day and opening them only after dark.
But they can do this only if they have evolved a way to filter out and fix the necessary carbon dioxide at night and use it by day for photosynthesis.
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