Growth in General 21 



parts of the growth cycle— from primordium to flowering, from flowering 

 to the end of exponential growth, and from this point to growth cessation. 



Growth in plants has usually been studied in the higher forms because 

 of their generally larger size and the greater ease with which observations 

 can be made upon them. Some lower plants, however, offer good oppor- 

 tunities for growth studies. Borriss (1934a) found that growth is not 

 evenly distributed in the sporophore stalk of Coprinus but is progressively 

 more rapid toward the apex. This has been confirmed by Bonner, Kane, 

 and Levey (1956; Fig. 2-6), who find that, after the early stage, growth is 

 accomplished chiefly by elongation of the cells of the hyphae. By dusting 

 the tips of young sporangiophores of Phycomyces with starch grains and 

 recording changes photographically, Castle ( 1958 ) has analyzed the 

 distribution of growth here, both as to longitudinal and circumferential 

 increase. The ratio between these two components is not constant but 

 changes with location on the sporangiophore. 



Brown, Reith, and Robinson ( 1952 ) examined the mechanism of growth 

 in plant cells, both in intact organs and by culture of isolated fragments. 

 Lindegren and Haddad (1954) found that in yeast cells growth rate is 

 constant and that it begins and ends abruptly, thus differing from growth 

 in most higher organisms. 



Physiology of Growth. The essential fact in growth is the increase in 

 amount of the various components of the organism. This results from the 

 self-multiplication of its essential portions, the genes and their basic 

 constituents, the nucleic acids. Everywhere syntheses are involved. This 

 general field is closer to physiology than to morphogenesis. Also essen- 

 tially physiological are problems concerning the rate and duration of 

 growth. These traits may be affected by many factors, some in the genetic 

 constitution of the plant and others coming from its environment, such 

 as temperature, light, water, and chemical substances of many kinds. To 

 consider these aspects of growth would require much space and is outside 

 the purpose of the present volume. The physiology of plant growth has 

 been frequently discussed, as by Thimann ( 1954 ) . 



It is not growth itself that is of morphogenetic importance but its 

 relative distribution, for this is what determines form. Richards and 

 Kavanagh (1945a) call attention to the fact that a study of growth by 

 geometrical changes alone, as is commonly done, does not tell the whole 

 story. Density (mass per unit volume) and volume may be increasing at 

 different rates in different regions. The forces of stretching and compres- 

 sion that result may affect the distribution of growth. Under the discus- 

 sion of various factors in the latter part of this book, growth and its con- 

 trol will from time to time be mentioned, but as part of a larger problem. 

 This problem is the development of a specifically formed and organized 



