The Cellular Basis of Growth 37 



a number of conifers, it does not reach a constant size but fluctuates rather 

 widely, falling and rising in cycles, perhaps climatic ones. Laing (1948), 

 Bissett, Dadswell, and Wardrop (1951), and Bannan (1954) report that 

 tracheid length tends to be less where the growth of the tree in diameter 

 is rapid, presumably because of the more frequent pseudotransverse divi- 

 sions of the cambial initials. Tracheid length is largely determined by 

 length of the cambial initials ( Bailey, 1920fr ) . 



Dicotyledonous woods follow the same general pattern as conifers but 

 the situation is more complex because of the greater variety of cell types 

 (Desch, 1932; Fig. 3-9; Kaeiser and Stewart, 1955). Fibers may increase 

 considerably in length over their cambial initials (Chattaway, 1936) but 

 vessel segments do not (Chalk and Chattaway, 1935). In storied woods 

 neither fibers nor parenchyma cells show any tendency to increase in 

 length from the pith outward (Chalk, Marstrand, and Walsh, 1955). In 

 all growth rings of pine, Echols (1955) finds a close correlation between 

 the fibrillar angle in the cell wall and tracheid length. The subject of fiber 

 length in woody plants has been reviewed by Spurr and Hyvarinen 

 (1954b). 



EXPERIMENTAL STUDIES 



The division and enlargement of cells are essentially problems in the 

 physiology of growth, a subject too extensive to discuss here in any detail. 

 Much experimental work has been done, however, on certain aspects of 

 cell growth which are of particular morphogenetic interest and which it 

 will be profitable to review briefly. 



The role of growth substances (Chap. 18) is particularly important. 

 Auxin was first recognized because of its stimulation of cell enlargement, 

 and in many cases it also affects cell division. Other substances are effec- 

 tive here. Jablonski and Skoog ( 1954 ) observed that the cells of tobacco 

 pith tissue in culture did not divide even under optimum amounts of 

 auxin unless extracts from vascular tissue, coconut milk, or certain other 

 things were added. This suggested that a substance specific for cell divi- 

 sion but different from auxin was here operative, and such substances, 

 the kinins, are now recognized. Gibberellin especially influences cell size. 

 Wound hormones induce division in many mature cells. The presence of 

 vitamin C seems to be related to cell expansion (Reid, 1941). Lutman 

 (1934) assembled a mass of data on the effects of various inorganic sub- 

 stances on cell size. The stimulating and inhibiting influences of these vari- 

 ous chemical factors are key problems in the physiology of develop- 

 ment. 



Metabolic factors are also important. Oxygen consumption is related to 

 cell division (Beatty, 1946). Interesting observations here have been re- 



