A modified sedimentation system for counting algae with an inverted microscope

The quantitative determination of small algae by the sedimentation of samples to which have been added Lugol's iodine followed by observation with an inverted microscope is well known and has been clearly described (LUND, KiPLING & LE CREN, 1958) . For about twelve years, modifications of the standard technique have been used and taught in the Botany Department of Royal Holloway College. The most important modification, which I shall refer to here as the "R.H.C . split-tube technique", has been described in various forms independently, elsewhere and more than ten years ago (DAWSON, 1960 ; LOVEGROVE, 1960) . However, despite the clear advantages of the modified method and its publication, I know of few laboratories that have adopted it . It would seem, therefore, in order and worthwhile to fully describe our standard method . Each sedimentation system consists of three parts (Fig . 1) . The upper part, constructed entirely of perspex, has a basal portion about 7 .5 x 2 .5 cm, with a central hole 9.5 mm in diameter. These dimensions are not critical but must match up with those of the other parts and the thickness should be about 3 mm . Symmetrically and permanently fixed with araldite, or fused on with perspex cement (perspex dissolved in chloroform), to this plate is a cylinder of perspex, cut from tube, which is 9.5 mm in internal diameter, 13 mm in external diameter and usually 3 .0 cm high . The middle part is similar to the upper but has no cylinder and has a shallow step fused onto the underside at each end . The third part consists simply of a No. 1 or No . 0 21 mm square cover slip. Before use all parts should be scrupulously clean . Immediately before the sedimentation procedure the underside of the upper part