Quantitative Indices of Succulence

Different indices have been proposed in the literature to describe quantitatively the capacity of a plant or a plant organ to store water, i.e., to quantify its succulence. 

Succulence reflects a tendency for reducing the surface of a plant relative to its volume (see Figs. 1.6, 1.10). If we assume a perfect sphere (perhaps the ideal succulent form) with a total volume of 1000 cm, the diameter would be 12.4 cm with a total surface area of 484 cm. A disc-like leaf of 1000 cm volume and 1 mm thick would have a total surface area over 20,000 cm. Hence, the disc shape will have a surface area some 41 times greater than the spherical shape of the same volume. Thermal consequences of the spherical shape for CAM plants will be discussed later (Chap. 5.3.6). The surface volume ratio of a plant or a plant organ, as well as its water-storing capacity, is quantitatively described by an index called Oberflachenentwicklung (i.e., surface expansion see Walter, 1926 Larcher, 1973)— where 

the fresh weight approximates the plant water content, assuming that 1 g fresh weight is equivalent to cm since the plant volume is largely a function of the water content. Some computations of surface expansion are given in Table 2.1. As can be seen from the table, surface expansion is determined to some extent by the water status of the tissue (see Walter, 1926). 

An alternative index of the water-storing capacity was introduced by Delf (1912). This index, termed degree of succulence (S), is 

S values of typical succulents are in the range of 5.1-14.9 g dmcompared to nonsucculents with values of 0.1-1.2 g dm .  

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