24 Growth 



in regeneration is difficult to explain as a result of the interaction of essen- 

 tially independent units. The cellular society, if it is one, must have a 

 strong central government which regulates the activities of its individual 

 members. A certain amount of self-differentiation undoubtedly exists, in 

 which a given organ or structure, once its development has begun, 

 proceeds more or less independently of the rest, but the parts are usually 

 interdependent. The problem of organization, the central one for biology, 

 can be attacked more hopefully by a study of organized systems as 

 wholes than simply of the units of which these are composed. 



There are obviously considerable differences in the degree and level of 

 organization. In plants with indeterminate growth, especially in some of 

 the lower groups, the "individual" is little more than a colony of cellular 

 individuals, which are so nearly independent that if isolated they 

 will produce new plants directly. Among higher forms it is much more 

 closely organized. Even here one can hardly tell, in types such as straw- 

 berries and many grasses, for example, which spread by stolons or 

 rootstocks, how much should be regarded as a single individual. In many 

 cases, however, growth is determinate, the number of parts is relatively 

 constant, and the individual is a distinct and specific thing. In no plants 

 does it reach the high level of organization that most animals display. 



In support of the concept that the organism is the developmental unit, 

 one may point to the many cases in plants where, as in the alga Caulerpa, 

 a very considerable degree of differentiation occurs into "roots," "stems," 

 and "leaves" but where there are no cellular boundaries at all. The whole 

 plant is a coenocyte, a simple mass of cytoplasm in which great numbers 

 of free nuclei are embedded or move about. In other algae where the 

 general character of the plant body seems to be similar to this, some 

 species have uninucleate cells, others multinucleate ones, and others are 

 entirely coenocytic, with no cell walls save where reproductive organs 

 are formed. In most of the true molds, or Phycomycetes, the hyphae are 

 multinucleate and not divided into cells, and this is true of certain of the 

 higher fungi also. In some other algae and fungi the partitions across the 

 filaments are incomplete and have a central perforation through which 

 cytoplasm can flow, so that there is no true cellular structure. In the 

 developing endosperm of the higher plants there is usually at first a 

 large number of free nuclei in a mass of cytoplasm, but these gradually 

 become separated from each other by the growth of walls. 



The difference between these two views of the relation between the 

 cell and the organism is of much importance for morphogenetic theory. 

 The individual cells are certainly significant, particularly in physiol- 

 ogy, and their presence makes possible much useful analysis of de- 

 velopmental processes, but just how a group of cells develops into an 

 organism still remains the central problem. 



