2 
INTRODUCTION 
them in a satisfactory manner according to their structural 
likenesses, or affinities, have continued with gradually increas- 
ing success for many years ; this line of study constitutes the 
science of systematic botany , classification , or taxonomy, and 
is based on comparative morphology. This in turn has 
itself been aided by the microscopic study of the internal 
structure of plants — anatomy , and minute anatomy or 
histology — especially in the earlier embryonic stages ( embry- 
ology ) of the life-history. 
Mainly through the work of Charles Darwin, it has 
in recent times been admitted that an evolution of the 
vegetable kingdom from a few simple to many complex 
forms has occurred and is still proceeding, and that struc- 
tural affinity is really an expression of relationship by 
descent. A complete revolution has thus taken place in 
the science. Taxonomic study now aims not merely at 
classification of now living forms, but also at tracing out 
their past descent from other forms, or phytogeny . Hence 
too, the study of fossil botany , or palceobotany , has of recent 
years become of great importance. 
Morphology has participated in the change of view, 
though only within the last few years. It is now clearly 
recognised that it too must be comparative, and based upon 
phylogeny, and that the classification of the organs or parts 
of plants is often almost as difficult as that of the plants 
themselves and involves the same phylogenetic consider- 
ations. At the same time it is recognised that the structure 
of an organ is intimately related with its function, and that 
change in the latter usually involves change in the former, 
and vice versa. Hence the modern morphology is studied 
largely in connection with physiology — the investigation of 
the functions of plants and their constituent parts or organs. 
Closely connected with physiology is ecology or natural 
history , the study of the functional, competitive, and other 
relationships of plants to their physical environment and 
to one another, and this throws very great light upon 
the meanings of the morphological features of plants. 
A very important proximate cause or mechanism of 
evolution is the variation which is so marked a feature 
of all plants and parts of plants ; the study of this is now 
coming into great prominence, as there are two rival theories 
