Symmetry 179 



the beginning, an organized system which grows at the tips of these 

 branches that constitute the "rhizome," "leaves," and "roots." These sys- 

 tems are symmetrical, either radially as in Bryopsis, or dorsiventrally as in 

 Caulerpa. In all these cases the origin of symmetry obviously is not re- 

 lated to planes of cell division or to other aspects of a multicellular sys- 

 tem but is dependent upon the behavior of the entire protoplasmic sys- 

 tem. 



Origin of Symmetry in Cellular Systems. In cellular plants, the origin 

 of symmetry can be traced more readily because it is expressed in the 

 division, growth, and relationships of cells at meristematic regions. 



In simple colonial forms like Pediastrum there is a regular sequence 

 of cell divisions from which a symmetrical plate of cells arises. In manv 

 algae with an indeterminate thallus, growth is controlled bv a large 

 apical cell. The origin of branches and the whole pattern of symmetry 

 are determined here. In simple two-dimensional thalli, the apical ceil 

 cuts off a daughter cell, first on the right-hand side and then on the 

 left, to form the so-called pendular symmetry. In most leafy liverworts 

 and mosses there is a pyramidal apical cell with three faces, and from 

 each of these, in regular succession, a daughter cell is cut off. The origin 

 of leaves is related to these faces, and in the simplest cases there are three 

 rows of leaves produced directly by this apical cell. 



In ferns and Equisetum, however, which also grow by a three-sided 

 apical cell, there is usually no relation whatever between the phyllotaxy 

 of the shoot and the configuration of this cell. In the seed plants there is 

 no single apical cell and no evident relation between the spiral pattern 

 of symmetry and any visible structures in the meristem. It seems clear that, 

 in all except the simplest plants, the origin of spiral symmetry is not re- 

 lated to cellular configuration at the meristematic region but must have 

 its basis in the entire embryonic mass. 



Dorsiventral symmetry in most cases is not established at the meristem 

 itself but has its origin in changes which arise later. Almost all meristems 

 or terminal embryonic regions are radially symmetrical. Dorsiventrality 

 may arise from these in the process of normal development. This is some- 

 times due to the influence of external factors such as light or gravity. It 

 is sometimes the result of position in the general plant body, as when a 

 branch becomes dorsiventral in symmetry. It is sometimes associated with 

 particular stages in the life cycle. In plants that are dorsiventral through- 

 out the mature plant body the seedlings are usually radial. In Hedera, the 

 vegetative stage of the life cycle is dorsiventral but the flowering shoots 

 are radially symmetrical (p. 213). Such changes are evidently due to 

 alterations in the internal environment. 



Such modifications of symmetry, particularly the change from the 

 radial to the dorsiventral type, involve not local regions but the entire 



