178 The Phenomena of Morphogenesis 



and constitutes a scorpioid cyme. The vetches and some other legumes 

 are less extreme cases, and there are many others. The flowers of such 

 dorsiventral inflorescences may themselves be radially symmetrical. 



Physiological Dorsiventrality. Dorsiventrality is manifest in physio- 

 logical activity as well as in structure, though usually not so obviously. 

 Plagiotropic roots, shoots, and other organs assume this position presum- 

 ably because of specific distribution of growth substances in the growing 

 tip such that the pull of gravity is counteracted and growth maintains 

 either a horizontal course or one at a given angle to a vertical axis. In 

 cases where the first division of a cell sets apart two different daughter 

 cells, as in the first division of the egg of Fucus, there is clearly a physio- 

 logical difference between the upper and lower halves. Indeed, the dif- 

 ferentiation of root and shoot in the embryonic axis, with the radical 

 differences in activity of these two poles, may be looked upon as an 

 example of physiological (and morphological) dorsiventrality. 



In leaves of certain water plants, externally alike on both surfaces, 

 Arens ( 1933 ) has presented evidence that the physiological activities at 

 the two surfaces are unlike, materials from the environment entering 

 through the lower surface and waste products (chiefly carbon dioxide) 

 being given off from the upper. What the mechanism of such physio- 

 logical dorsiventrality may be is not known, but bioelectrical differences 

 (p. 361 ) are perhaps involved. 



DEVELOPMENT OF SYMMETRY 



The causes of organic symmetry are not well known, but in endeavoring 

 to find them it is first necessary to determine how these relations actually 

 arise in the process of development. 



The Origin of Symmetry in Coenocytic and Colonial Systems. Sym- 

 metry is by no means confined to cellular structures. From the Plas- 

 modium of myxomycetes, formless and unsymmetrical, there arise spe- 

 cifically formed and radially symmetrical fruiting bodies of great variety. 

 Here the morphogenetic process may be seen in one of its simplest ex- 

 pressions, as the sporangium is molded from the plasmodial mass. 



Even more remarkable are those slime molds belonging to the 

 Acrasiaceae (Dictijostelium and its allies, p. 223) where the vegetative 

 body is a single myxamoeba. At the end of the vegetative period thou- 

 sands of these come together into a pseudoplasmodium but do not fuse. 

 This colony, after some migration, settles down and develops into a 

 radially symmetrical stalked sorocarp. 



More closely resembling the bodies of the higher plants but still with- 

 out cellular boundaries are the coenocytic members of the algae and 

 similar groups. Here there is no formless mass of protoplasm but, from 



