204 
MORPHOLOGY OF MEMBERS. 
tionships are sometimes reversed. When the growth is unlimited, its products 
along the axis are usually constantly repeated, the segments formed one after 
another are similar, the lateral members that spring from them (branches, leaves, 
lateral roots, &c.) are uniform, or they exhibit in their development a repeated 
alternation, as, e.g. in Moss-stems, rhizomes of Equisetum, primary stems of Coni- 
fers, &c. When, on the contrary, the growth along the axis is limited and definite, 
the resulting segments are dissimilar, and their outgrowths exhibit progressive 
changes (metamorphosis). This occurs in most leaves, the basal portions of which 
are usually strikingly different in form from the parts nearer the apex; it occurs 
also in the stems of Angiosperms with terminal flowers, which commence, for 
instance, with the formation of radical leaves, proceed to that of foliage-leaves, and 
then, through the bracts, pass over into the production of floral leaves, closing with 
that of carpellary leaves. 
Axial growth is always limited when true dichotomy occurs at the apex; on 
the other hand, bifurcations repeat and continue the mode of development of their 
common basal portion (as in Fucus or Selaginella\ although individual branches 
may terminate their growth without dichotomy by producing fruit. 
(4) If an axial longitudinal section is imagined to pass through a member, 
the conformation right and left may be similar, like the right and left halves 
of the human body. If the two halves are so similar that the one is a reflected 
image of the other, they are sy??i?ne/rical, and the dividing plane between them 
is called a plane of symmetry. In this strictest sense symmetry is very rarely 
found in plants (most nearly in many flowers and stems with decussating whorls); 
and accordingly the term is constantly employed in a laxer sense. Two, three, four, 
or a larger number of symmetrically dividing planes often pass through a member 
(a branch or root), all of which intersect in the axis of growth. Such members 
are called poly symmetrical ; so-called ' regular ' flowers, stems with alternating 
whorls, and most roots, are polysymmetrical. If, on the contrary, it is possible 
to imagine only one symmetrically dividing plane, as in the flowers of Labiatse 
and Papilionaceae ^, in stems with opposite pairs of leaves, where the median 
plane of the two rows of leaves is at the same time the plane of symmetry, in 
the thalloid shoots of Marchantia^ and in most leaves, the object is vionosymmetrical^ 
or simply symmetrical. Monosymmetry is however only a particular case of the 
ordinary bilateral structure, which consists in the processes of growth being 
similar to the right and left of an axial longitudinal section, although the 
two halves of the member do not lie exactly opposite to one another like 
reflected images. Thus, for example, the oblique leaves of Begonia are not 
symmetrical, although bilateral; the one half to the right of the mid-rib of the 
lamina is larger and of somewhat different shape to the other half to the left of 
the mid-rib; and the same is the case with the elm. A branch with alternating 
leaves in two rows is also simply bilateral without being monosymmetrical ; 
if it is divided at right angles to the common median plane of all the leaves, 
the two halves bear each one row of leaves ; but the one is not the reflected image 
^ A. Braun calls monosymmetrical flowers zygomorphic, an expression which is also elsewhere 
interchangeable with monosymmetrical. 
