Bentham and Hooker's system of classification. I have chosen the representative plants from the different parts of the desert and have examined fifty orders, 125 genera and 165 species. In dealing with the individual order, I have described the specific characters, the structure of the leaf and the structure of the axis and when necessary I have appended a general review of the order. I have omitted the structure of the root, as generally it is not available in herbarium specimens. The specific characters that are considered, are just those that will be useful in the diagnosis of the species. The different structures are considered in detail under the heads of "The structure of the leaf" and "The structure of the axis". The characters which are common to both the leaf and axis are usually dealt with jointly under the structure of the leaf as they are more prominent in the leaf.
I have embodied the interpretations of the various modifications of the different tissues in the descriptions of the structures of the leaf and the axis, and in the general review of the order, so that the descriptions can be said to contain at once an account of the anatomy and physiology of the plants in question. The concluding remarks at the end of the principal part of the work serve as a concise and complete summary of the treatise.
{e) Method. — As regards the method employed in preparing the herbarium material for the section work, small pieces of the leaves and the axes were soaked in water for about two hours and then hardened in formalin alcohol for about a day. The sections which were cut in 90% alcohol were placed, before mounting in glycerine, in a few drops of lactic acid on a slide and slightly warmed over a burner, so that the cells of the different tissues might expand to their proper dimensions.
(f) A suggestion. — The idea of employing micro-anatomical and micro-chemical characters for systematic purposes originated in times as early as those of Linnceus, and has been repeatedly put forward by several botanists, Radlakofer being considered as the founder of the anatomical method. It would be presumptuous on my part to discuss the merits of the method for systematic purposes; but from what little I have observed in my study of the physiolo- gical anatomy ol the desert flora, I have grounds for believing that the study of Systematic Botany would considerably progress were more attention paid to anatomical characters than has heretofore been done. The method is no doubt laborious and a systematic botanist would be loth to adopt it, owing to the huge amount of the material submitted to him for identification from all sides. I admit this difficulty and suggest that the anatomical characters should be used at least as confirmatory evidence in establishing genera and species.
