350 V. THE NATURE OF EVOLUTION 



probably because the environmental condition under which the germi- 

 nal cells were produced is changed to become favourable for the for- 

 mation of somatic cells. However, the condition or the factor which 

 formed the somatic cells may usually be removed when the somatic 

 cells are once completed, so that somatic cells are existing normally 

 under the condition favouring the primitive form, but without return- 

 ing to the original, just as water can sometimes be present as such 

 under 0°C. without being changed to ice. Therefore, the completed 

 somatic cells may be in a metastable state with a great tendency to 

 change into the primitive structure. In this train of thought it is easily 

 explained that a simple stimulus such as cutting is sufficient to cause 

 the recovery of the primitive structure just as super cooled water 

 is readily changed into ice on the application of a slight stimulus. 



The capacity of regeneration seems to be especially great in plants. 

 This indicates that some somatic cells of plants have particularly 

 strong tendency to return to the primitive state. On the other hand, 

 in higher animals regeneration occurs only to a small extent and even 

 with difficulty. This may be due to the presence of some mechanism 

 preventing the recovery of the primitive structure. In the majority of 

 animals, gonad-forming cells are differentiated for the purpose of the 

 production of primitive cells, /. e. germ cells, whereas, in plants, 

 primitive cells including germ cells appear to be generated with ease 

 from usual somatic cells not differentiated for the purpose. This fact 

 also shows that in plants the structural reduction to the primitiveness 

 can easily take place. Again, it is known that a variety of physical, 

 chemical and biological agents readily induce pathological growth in. 

 plants ; in general, cells of primitive structure have the capacity of 

 vigorous growth, while pathological growth is associated with the 

 primitive character as with cancerous cells. Of physical agents in- 

 ducing such a pathological growth mechanical injury is well known. 

 Diseased growth considerably beyond that necessary for healing fre- 

 quently occurs in plants following injury. A large number of chemi- 

 cals are capable of inducing more or less diseased growth. This is so 

 common that one can expect diseased growth to follow the application 

 of almost all chemicals at a concentration which induces injury but 

 does not kill. Also living agents, such as bacteria, fungi and nema- 

 todes, cause various kinds of pathological growth (22). 



Thus, it may be concluded that plant cells are strongly inclined 

 to be reduced to primitiveness following the application of a stimulus 

 disturbing the protoplasm structure, in the same way as the super- 

 cooled water tends to be readily frozen on the application of a disturbing 

 stimulus. This character is especially manifest in plants because it 

 is a beneficial for them ; namely, since this character enables plants 



