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Figure 1.7

The shapes of phytoplankters: log/log plot of maximum linear dimension (MLD) versus nominal s/v of individual phytoplankters (data in Table 1.2). Similar morphologies are grouped together (I, spherical cells; II, spherical colonies; III, squat ellipsoids and cylinders, of which IV are exclusively centric diatoms; V, attenuate cells and filaments; VI, coenobia; VII, bundles of filaments of Anabaena or Aphanizomenon); C (for Ceratium) and S (for Staurastrum) identify shapes with large protuberances). The vertical dotted lines define the distributions of most marine phytoplankters according to Lewis (1976). Redrawn from Reynolds (1984a).

represented these modifications by plotting the maximum linear dimension (MLD) of the unit against its surface-to-volume ratio. His approach is followed in the construction of Fig. 1.7, in which the relevant data from Table 1.2 are plotted. The diagonal line is a geometric boundary, representing the diminution of s/v of spheres against the increment in diameter and, indeed, upon which the spherical unicells (marked I) and colonies (II) are located. All other shapes fall above this line, the further above it being the more distorted with respect to the sphere of the same MLD. The broken dotted lines bound the s/v ratios of non-spherical forms. All the ellipsoid shapes (III, such as Mallomonas, Rhodomonas), squat cylinders (IV, including Cyclotella and Stephanodiscus spp.), attenuated needle-like cells and filaments (V: Monoraphidium, Closterium, Aulacoseira, Planktothrix) fall within this area. So do the coenobial forms comprising individual attenuated cells (VI, e.g. Asterionella, Fragilaria) and the unicells with significant horn-like or arm-like distortions (Ceratium, Staurastrum, individually identified). The plot backs the assertion that the attractive and sometimes bizarre forms adopted by planktic freshwater algae are functionally selected.

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