DIRECTIONS OF GROWTH. 



183 



member ; this can indeed be determined from a very small fraction of it • in a 

 Mammillaria, a INIelocactus, or a Cereus, it is just as easy to determine the longitudinal 

 axis of growth in early youth, when these Cacti are often as thick as they are long, 

 as it is later when they are much longer than thick. This is also the case in the 

 abbreviated axis of bulbs, in many tubers and corms (as Crocus), and in fruits, 

 like those of many gourds, whose diameter is much greater than their length. 



The growth of roots and stems in the direction of their longitudinal axis is 

 generally unlimited, that of leaves and hairs mostly limited, although these rela- 

 tionships are sometimes reversed. When the growth is unlimited, the formations 

 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, commencing, for instance, 

 with the formation of radical leaves, proceeding to that of foliage-leaves, and then, 

 through the bracts passing over into the production of floral leaves, closing with 

 that of the carpellary leaves. 



Axial growth is always limited when true dichotomy occurs at the apex ; on the 

 other hand the 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 

 one member (a shoot or root), all of which intersect in the axis of growth. 

 Such members are called polysymmetrical ; 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 Labiatje and Papilionaceae \ in stems with leaves arranged in two rows, 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 

 monosymmetrical, or simply symmetrical. Monosymmetry is however only a par- 

 ticular case of the ordinary bilateral structure ; it consists in the processes of 

 growth being perfectly similar to the right and left of an axial longitudinal section, 



^ A. Braun calls monosymmetrical flowers zygomorphic, an expression which is also elsewhere 

 interchangeable with monosymmetrical. 



