CHAPTER 7 



Symmetry 



The presence of an axis, so generally characteristic of the form of body or 

 organ in animals and plants, is manifest not so much as an actual material 

 structure but as an axis of symmetry, a geometrical core or plane around 

 which or on the two sides of which the structures are symmetrically dis- 

 posed. One of the most obvious manifestations of organic pattern in 

 living things is this symmetrical arrangement of their parts. 



Symmetry is evident in both external form and internal structure. Lat- 

 eral roots arise from a primary root in two, three, four, or more equally 

 spaced rows. Leaves are symmetrically disposed around the stem in a 

 phyllotactic spiral. Floral diagrams, both transverse and longitudinal, also 

 provide good examples of axial symmetry, though here the axis is usually 

 much shortened. 



Symmetry is equally conspicuous in internal structure. The cross sec- 

 tion of almost any vertical plant axis shows symmetrical arrangement of 

 its tissues, both primary and secondary. Even single cells, especially when 

 they possess a considerable internal diversity like those of Spirogyra, are 

 symmetrical. 



In horizontal organs the simple radial type of symmetry characteristic 

 of vertical axes is replaced by a dorsiventral one where the two halves on 

 either side of a vertical plane of symmetry are alike. Many prostrate stems 

 and most leaves are examples of such dorsiventrality. Sometimes one type 

 of symmetry may be changed to the other by modifying the orientation 

 of the structure to light or gravity. In other cases the pattern of symmetry 

 is inherited and cannot be influenced by environmental factors. 



Many structures are in themselves asymmetric. In leaves of Begonia 

 and elm, for example, the portions on either side of the midrib are usually 

 quite unlike; and there are marked internal asymmetries, as when two 

 daughter cells are dissimilar. In most cases of this sort, however, the 

 asymmetry proves to be part of a larger and more complex pattern which 

 is symmetrical. 



Symmetry is often more conspicuous in embryonic structures or meri- 

 stematic regions than at maturity, and some of its most remarkable ex- 



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