330 THE EVOLUTION OF THE METAZOA 



colonies, and the development by way of a polymerization of 

 karyons and a differentiation of the cytoplasm. The situation in 

 Protophyta is clear because they depend entirely on a gaseous 

 food, salts dissolved in water and the sun rays that penetrate the 

 exterior of Protophyta. Their units had soon developed firm 

 cell walls and they had shown from the very beginning 

 an inclination to form coenobia and colonies (cormi). It 

 is certainly characteristic that in plants the transition from 

 the monocellular to a polycellular state had occourred seve- 

 ral times, as this is now generally believed by botanists. 

 The Vo/pox-]ine. which has been so frequently misused as a 

 pattern must be mentioned here as a special case. Volvox is 

 characterized by the fact that it has remained a freely-moving 

 form even after it had begun to form special "colonies" and 

 after it had made the first steps towards a differentiation. 

 Something similar had also taken place in the Dinoflagellata. 



The subsequent evolution of the polycellular plants had been 

 very slow and it did not make any important progress in the 

 direction of an integration (individualization) and of a differen- 

 tiation (the formation of tissues and of organs). Still we should 

 not attribute all these facts as being due to the colonial origin 

 of plants. The whole regime of metabolism has also had an 

 important role in this connection, as well as the immobility of 

 plants (the fact that they are attached to their substratum). A 

 local and partial mobility was achieved in a different way than 

 in the case of the Protozoa and of the Eumetazoa; this was due 

 to the fact that no muscles or nerves have ever been developed 

 by plants in spite of their ability to contract and to conduct 

 irritation. 



It has already been mentioned that in Metaphyta a signi- 

 ficant formation of syncytia can not infrequently be observed 

 (Caulerpa and its relatives, some tissues of higher Metaphyta 

 which function as vessels, etc.). The example of Siphonales 

 shows that no special plant type has ever been developed by 

 Metaphyta by way of secondary syncytia which would include 

 new elements that could be considered progressive. 



