With 5 figures in the text) SUMMARY 'Relative frequency' recorded by point quadrats measures not the actual area of foliage but the area projected in the direction in which the quadrat lies. Accordingly the relative frequency varies both with the slope of the foliage and also -when inclined quadrats are used -with the inclination of the quadrat. A theoretical study reveals that variation in relative frequency resulting from difi^erences in foliage angle is greatest for vertical quadrats, is considerably reduced when (as suggested by Tinney, Aamodt and Ahlgren) quadrats are inclined at 45°, and is least when quadrat inclination is 32.5°. Accordingly the usual, vertical position for point quadrats is the worst possible one, since it results in the most erroneous estimates of percentage contribution (area basis); while with quadrats inclined at 32.5° errors are greatly reduced and are of an order acceptable in general survey work.

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The total area of foliage per unit area of land can be estimated from the number of contacts with foliage made by point quadrats inclined at appropriate angles, and the distribution of this foiiage area with height above ground be estimated by recording the heights at which the contacts occur. These estimates, unlike those based on vertical and horizontal quadrats, are not depressed by variability in foliage angle (inclination of foliage to the horizontal). The foliage angle can be estimated by comparing the numbers of contacts made by quadrats inclined at two different angles, but this estimate is less satisfactory than one based on vertical and horizontal quadrats. Accurate estimates of both foliage area and foliage angle can be obtained by using a combination of vertical, inclined, and horizontal quadrats. This paper also discusses the application of point quadrat methods to stems as well as leaves, and considers certain practical aspects of point quadrat use.

Summary ‘Relative frequency’, yielded by point quadrat analysis of vegetation, is a measure of the area of the foliage in vertical projection. Because this projected area varies according to the foliage angle (i.e. the angle between foliage and horizontal), values of relative frequency fluctuate as attitudes change with environmental variation; also, misleading estimates are obtained when comparisons are made between species of different habit. By using horizontal as well as vertical point quadrats it is possible to estimate the foliage angle and the foliage denseness (i.e. total area of foliage per unit volume of space). If the frequencies of contacts with foliage per unit length of point quadrat are F90 and F0 for vertical and horizontal quadrats respectively, foliage angle (α) is given by tan α/2(F0/F90) and foliage denseness (F) by F=F90 sec α. The vegetation is analysed within horizontal layers. An apparatus is described which can give the required data, and which avoids the errors inherent in point quadrat methods that use needles of appreciable thickness. The total area of foliage per unit area of ground can be obtained by summation of F for all layers. Trials show that the method is practicable in the field, and that it gives values of α and F which reflect the character of the vegetation. Values of F obtained by this method agreed with direct estimates for the same vegetation within 2.5%. Some assumptions made by the method are discussed; provided that certain precautions are taken, the errors resulting from these are small or none.

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