568 
MR. F. O. BOWER ON THE COMPARATIVE MORPHOLOGY OF THE 
the most important points, and that, however much those relations may apparently 
be disturbed by various localisation of subsequent growth, the changes thus induced 
are to be regarded as but of secondary importance. 
In dealing with the leaf this principle is not kept so clearly before the mind, and 
the explanation is doubtless to be found in the fact that the intercalary growth is 
usually localised, at or near the base of the leaf, at an earlier period than is the case 
in the stem. Current morphology still retains those obvious distinctions of sheath, 
petiole, and lamina, as coordinate structures, independently of the fact that the most 
salient and distinctive characters used in distinguishing these several parts one from 
another result from processes of growth in transverse and longitudinal directions, 
which, in the treatment of axes, would be freely admitted to be of but secondary 
importance, and certainly would not be allowed to take precedence of those phenomena 
of branching upon which the morphological treatment of the shoot is primarily based. 
In the leaf precedence is given to the more obvious results of intercalary growth, 
while the branching of the leaf is treated as of but secondary importance. This 
inconsistency demands further investigation. It must be ascertained by a comparative 
study of the leaf in forms acknowledged to be lower in the scale, whether there he 
sufficient reason for treating the leaf differently from the stem. It may be stated at 
once that in my opinion the comparative study of leaves of the lower vascular plants, 
which will be detailed below, does not justify such a difference of treatment. 
Before bringing forward that mode of treatment of the leaf, which I shall propose as 
being more in accordance with the method of treatment of the axis, it will be neces¬ 
sary to say a few words on the subject of growth as affecting the external conformation 
of members. 4 ' Differences in direction, intensity, localisation, and duration of growth 
transversely to the organic axis of a member result in differences of its external form ; 
and the forms which result may be roughly ranged in three series, though these 
graduate by intermediate forms one into another : (1) When the growth is equal in 
all directions transversely to the organic axis, the member is cylindrical. (2) When 
the growth along one transverse diameter is a maximum, and that in a diameter at 
right angles to it is a minimum, a flattened member is the result. In both the above 
cases the growth (exclusive of secondary changes) is usually nearly simultaneous over 
the wdrole cross-section. (3) When the growth is at first more or less uniform over 
the whole cross-section, but is subsequently localised, and continued more rapidly at 
one or more points at its periphery, variously winged members are the result. These 
three modes of development, as well as transitional forms between them, are to be 
found exemplified both aniung axes and leaves ; but while the first is most common 
for axes, and most rare among leaves, the second and third are most common for 
leaves, and less usual for axes. It frequently happens that in one and the same axis 
there may be a gradual or even a comparatively sudden transition from one of these 
forms of development to another. Thus, in stems of Rhipsalis, Coccolobci, XylopliyUa, 
* Compare C. He Candolle, ‘ Theorie de la Feuille,’ p. 19. Geneve, 1868. 
