(1) The basic principle is the use of colours of an ecological value. Among the factors of the environment, only those which play a part in the water equilibrium of the vegetation will be shown and the combination of their colours will become representative of the ecology.
High temperature and long dry period will increase the evaporation. Rainfall and relative humidity 'will be favourable to the plant growth. Then a conventional, but natural choice of blue and violet shades for humidity, and on the other hand, red or yellow shades for heat and drought will provide all the coloration possibilities.
Six classes are defined for Temperature, Rainfall and Xerothermic index (length and intensity of the dry season). Each class has its conventional shade. In any place the climate is easily summarised into an ecological formula which will include for more precision the light factor, the vegetative period and the soil factor.
Soil factor is taken into account:
-either by its effect on the water balance. The colour is shaded accordingly, for instance, the mangrove, the vegetation of saline soils ;
-or by its limiting effect on the floristic composition of the vegetation types. For example, the lateritic caps. These features are shown by the superimposition of a symbol.
(2) Another originality of the method is the representation of "Series.of Vegetation". Various physiognomic stages of degradation of the same series are shown by means of lines, stripplings etc. But the same colour, representative of the general ecology of one series is used for all its serial stages.
Floristic releves are made in the field. Their comparison brings out the relationship between the successive stages of the series and leads to the most developed type possible under the present ecological conditions. This maximum stage, the least disturbed by biotic factors, is the "plesioclimax". It is the maximum vegetation to be expected under present conditions if human interference it excluded. Its characteristic and dominant species axe used to name the series.
The plesioclimax helps to appreciate the ecological-potentiality of the place and therefore will be of some guidance for the introduction of new species.
Since similar colours are used for similar climates in the world, the colours show homo-bioclimates under which grow similar vegetation, types even if their floristic composition is different.
Several examples are taken from vegetation maps of Tunisia, Algeria, and South India.