96 The Phenomena of Morphogenesis 



ginning of a discussion of these problems it will be useful to consider 

 some typical examples of the ways in which growth of one part or dimen- 

 sion is related to growth elsewhere or to the plant's various activities. 

 These will illustrate how plant forms arise and an integrated organism is 

 produced and will serve as an introduction to the fundamental problem 

 of morphogenesis, approached from many directions throughout this book 

 —the problem of biological organization. 



There have been many discussions of correlation in the literature of 

 plant development, and for some of them the reader is referred to the 

 works of East (1908), Harris (1909-1918), Love and Leighty (1914), 

 Murneek (1926), Goebel (1928), Thimann (1954b), and others. 



Correlations have been classified in many ways, as environmental, 

 physical, morphological, physiological, genetic, compensatory, or meristic, 

 depending on the characters and factors involved. For purposes of con- 

 venience in the present treatment, there will be grouped together, as 

 physiological, those correlations for which a physiological mechanism- 

 metabolic, hormonal, or other— seems to be operative and as genetic, those 

 which seem to depend primarily upon the genetic constitution of the 

 individual and its formative relationships and are thus produced by 

 mechanisms more deeply seated and obscure than the ordinary physio- 

 logical ones. 



PHYSIOLOGICAL CORRELATIONS 



Physiological relationships are of particular morphogenetic interest 

 since through an analysis of them the mechanisms for other types of cor- 

 relation may be discovered. The various factors concerned will be treated 

 in later chapters. The particularly important role of growth substances in 

 plant correlation has been discussed by Thimann ( 1954fo ) . 



It will be useful here to mention a few typical examples of physiological 

 correlation and to formulate some of the problems that they present. 



Nutritional Correlations. The simplest type of correlation is one which 

 depends on nutrition. A region that does not produce or contain food must 

 depend for its growth on one that does. Correlation of this sort between 

 root and shoot must obviously occur. The root-shoot ratio is a favorable 

 one in which to study correlation and the factors that modify it, and con- 

 siderable attention has been given to the problem. Kny (1894) cut off 

 part of the roots from growing seedlings and part of the shoot from others. 

 When a considerable amount of reserve food was still available in the 

 seed, loss of one part did not greatly affect the growth of the other. 

 Pearsall (1923), Keeble, Nelson, and Snow (1930), and others found 

 that removal of the seedling shoot sometimes actually stimulated growth 

 of the root, presumably because of reduced competition for food stored 



