Phytogeography, as the name indicates, is the scientific study of the geographical distribution of plants. It is a science combining botany and geography. The subject can be approached in at least two ways.

1. By studying the geographical distribution of assemblages of plants, namely plant communities. This is a phytosociological approach

2. By studying the geographical distribution of individual plant taxa. This is a phytogeographical approach in a restricted sense and it is the approach that this book adopts.

Both approaches are relevant and inter-related. Results obtained from phytosociological studies can throw light on phytogeographical problems and vice versa. The methods used are, however, quite different.

The basic material in phytosociological studies is the relevé, i.e. standardised descriptions of vegetation plots with lists of species and information about their relative quantities or abundances. Sets of relevés are combined into vegetation tables which can be analysed by different methods. One method is to attempt a vegetation classification, the approach used in classical plant sociology. Another approach is to consider the vegetation table as an information matrix which can be represented as points in a multidimensional space. By using different methods, invariably employing computers, relations between relevés and species can be studied. This is the ordination or gradient analysis approach (Gauch 1982).

The basic material in the phytogeographical approach in its restricted sense is data on the occurrences of individual plant taxa. These are summarised verbally in floras, indicating where the plant in question might be found. Little information can be presented in this way, however. An alternative approach is to make distribution maps of plant taxa. These can be made in several different ways.