THE SYSTEMATIC ANATOMY OF THE VEGETATIVE ORGANS OF THE ANGIOSPERMS
Authors: METCALFE C. R. 
Published: January 2008

Historically the study of the anatomical structure of the vegetative organs of the flowering plants for the purpose of classification has developed slowly owing to the rather wide gulf which at one time existed between taxonomy and other branches of botany. In recent years the subject has been given a new stimulus through a wider interest in wood structure, chiefly amongst foresters and timber technologists. Further stimulation has been caused by the formation in 1930 of an International Association of Wood Anatomists.

Satisfactory investigations in systematic anatomy must be based on accurately named material which is not always easy to obtain, especially from tropical regions. Reference collections of microscopical slides are as necessary to the systematic anatomist as are herbarium specimens to orthodox taxonomists. Representative slides made during important systematic investigations should be deposited at a recognized taxonomic centre so that they can be consulted by future workers. Arrangements to facilitate the exchange of slides and information between institutions where systematic anatomy is studied are important.

There is at present no purely scientific method of identifying or classifying botanical material by microscopical methods. Success is achieved, in practice, largely by memorizing anatomical features, by experience and intuition. Circumstantial as well as strictly botanical information is often helpful. Dichotomous and similar keys are useful, but of limited value when dealing with a large group of plants such as the whole of the dicotyledons. Reference is made to a multiple‐entry perforated card index system now in common use for the identification of timbers, and it is suggested that semi‐mechanical methods of this kind could usefully be applied to other branches of systematic anatomy or even to taxonomic studies based on exomorphic features. It is most important that characters selected for diagnostic purposes should be those which practical experience shows to be reasonably constant and not liable to modification by environmental changes.

The practical value of systematic anatomy cannot be fully assessed until much more descriptive work has been done. It is known that the method is of definite value in the interpretation of genera, and that it often affords evidence concerning the interrelationships of families. It is of less value in the interpretation of species and taxonomic units of still lower rank. Systematic anatomy is of economic importance, since it enables timbers, fibres, crude drugs and similar botanical material to be identified even when floral characters are lacking. The approximate affinities of sterile herbarium specimens may also be established by this method. A more complete knowledge of the systematic anatomy of present‐day plants would be of assistance to palaeobotanists.

Recent studies have shown how graded series of anatomical features in the secondary xylem of the dicotyledons and in the metaxylem of the vascular bundles of the monocotyledons can be used to indicate probable phylogenetic sequences. There are difficulties in comparing woody plants and herbs from this point of view owing to the small amount of secondary xylem in the latter. This fact sets a limit to the value of phylogenetic studies which are based on wood structure alone.

The basis of taxonomy is being broadened at the present time, and there seem to be reasonable prospects that, in the future, the facts of systematic anatomy, together with those of cytology, embryology, genetics and other branches of botany, will play an important part by helping to give a deeper significance to systems of'classification than they have done in the past.