PART II 



PHYSIOLOGY OF GROWTH 

 AND CONFIGURATION 



CHAPTER I 



GENERAL DISCUSSION OF GROWTH 



§i. Anatomical Relations of Cell Growth. — Microscopical observation of 

 the development of plant cells shows that three different stages of growth may 

 be distinguished. The growth of the cell begins with its formation by division, 

 this is the first stage of growth. The cell then begins to increase in size, thus 

 passing into the period of enlargement, which is the second 

 stage. Enlargement finally ceases, to be followed by 

 thickening of the cell wall through the deposition of new 

 layers of cellulose, and this constitutes the third stage of 

 growth. The last two stages are not entirely distinct but 

 merge gradually into each other, for deposition of new 

 layers of cellulose occurs simultaneously with the enlarge- 

 ment of the cell. Fig. 96, a cross-section through the 

 cambium region of the stem of the Scotch pine, shows all 

 three stages in the development of tracheides from 

 cambium cells. If all the cells of a tissue are in the first 

 or in the third stage of growth, the growth changes charac- 

 teristic of these stages are without effect upon the size of 

 the tissue mass. In considering a tissue, these two stages 

 may therefore be designated as stages of internal growth, as 

 distinct from the second growth stage, that of enlargement, 

 of which increase in the dimensions of the tissue or organ 

 is the most characteristic feature." 



Not only is a sharp distinction between the second and third 

 stages of growth impossible, as the author states, but the same is also 

 true regarding the first and second stages; a certain amount of en- 

 largement usually precedes each cell division in tissues that are ac- 

 counted as in the first stage. The three stages furnish a convenient mode of reference, how- 

 ever, to the corresponding portions of the continuous march of the growth process. The first 

 stage (called also the embryonic or formative phase) is mainly characterized by cell division, 

 the second (called the phase of enlargement) is mainly characterized by cell enlargement, 

 and the third (called the phase of maturation) is mainly characterized by thickening and other 

 alterations in the cell walls, frequently also by changes of other sorts. — Ed. 

 16 241 



Fig. 96. — Cambium 

 cells of Scotch pine, 

 showing transforma- 

 tion into tracheides. 

 The Roman numbers 

 denote the three stages 

 of growth. 



