ii54 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 oi 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 symmetrical^ 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 polysymmeirical ;■ 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. Labiatae 

 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 oi Mar-chaniia, and in most leaves, the object is monosymmetrical, 

 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. 



