20 
STRUCTURE AND FUNCTION 
evidently tends to greater efficiency in the performance of 
those functions. There is, in fact, a functional or physiologi- 
cal division of labour, accompanied by a structural or morpho- 
logical differentiation of organs. To investigate the functions 
of plants and their gradual differentiation and perfection 
is the province of physiology \ to do the same for the organs 
that of morphology, which seeks to know and interpret the 
structure and development of all organs, to trace and 
explain their origin, descent and modifications (i.e. their 
phylogeny ) and to group them according to their natural 
relationships to one another by descent. 
Not only do differentiations of structure and function go 
hand in hand, but also changes of these features, which 
may be brought about, for example, by change of climate 
or soil, or by other circumstances. In many cases we can 
see this change of structure or metamorphosis actually 
occurring in connection with a functional change, e.g. in 
Astragalus, where a leaf changes to a thorn, in Geum, where 
a style changes to a hook, and in the formation of fruits. 
Mutilation may bring about metamorphosis, as in Abies, 
or attacks of fungi, as in the formation of witches’ brooms on 
trees (Chap. III.). In most trees in cold climates, the last- 
formed exposed leaves in autumn are bud-scales, or leaves 
whose peculiar form is due to enlargement of the base and 
suppression of the blade of an ordinary leaf during its 
growth ; the function of these scales is to protect the bud 
during the winter. This leads on to the case of “successive” 
metamorphosis shown by such water plants as Littorella and 
Polygonum, where if a plant become submerged which was 
before exposed, it produces new leaves structurally and 
functionally suited to the new conditions, but does not 
alter the old ones. Here we can still see the change occur, 
but there are also innumerable other cases where we cannot 
do so, but where we find an organ performing a different 
function and possessing a different structure from those 
of the organ from which the general evidence shows it 
to be descended ; thus an organ that is descended from 
a true leaf may have the function and structure of a root, 
as in Salvinia. Other instances are the leaf-like roots 
1 Comparative physiology is as yet, however, in a very early stage. 
