248 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



Summary 



1. Anatomical Relations of Cell Growth. — The first stage, or phase, of cell growth 

 is that of cell division — this being also called the formative, meristematic, or embryonic 

 phase. After its formation by division each cell enters the second phase, that of 

 enlargement, in which it attains its mature size. After enlargement ceases various 

 changes occur leading to the condition of maturity, such as the thickening of walls, 

 etc., and these changes characterize the third phase, that of maturation. The three 

 growth phases cannot be sharply distinguished, however. 



Measurements of growth rates frequently refer only to the second phase, that of 

 enlargement, since this is the most easily studied by means of a millimeter rule, etc. 

 Microscopic methods are resorted to in studies of all three phases, growth in general is 

 frequently measured in terms of increase in weight, and chemical determinations of the 

 various substances formed are often employed, especially for studies of the third phase. 



2. Conditions Favorable to Growth. — Growth is a complex physiological process, 

 being the resultant of many component processes, and — like all other processes — it 

 cannot occur unless all the essential conditions are fulfilled. Some of the essential 

 conditions are internal (within the plant body), while others are external (in the 

 environment). One of the primary internal conditions necessary for cell enlargement 

 is turgidity, which is produced by osmotic pressure or imbibition pressure within the 

 cell. By this pressure the protoplasm is held against the cell wall and the latter is 

 more or less stretched. Cell enlargement is primarily due to osmotic and imbibition 

 pressures developed through the absorption of water, and to the resultant stretching 

 of the cell wall. New cellulose is deposited in the stretched wall and more stretching 

 occurs, with still further addition of cellulose, until the second phase of growth is 

 completed. In the third phase of growth the cell wall often thickens on account of 

 deposition of cellulose without further stretching. 



The importance of turgidity is shown experimentally by plasmolysis and the 

 recovery therefrom. Artificial osmotic cells, especially Traube's artificial cell (a drop 

 of gelatine solution in a solution of tannin), illustrate some of the phenomena of turgor 

 and plasmolysis. When enlarging plant organs or tissues are placed in properly 

 concentrated salt or sugar solutions a pronounced contraction results, due to the 

 removal of the turgor pressure within the cells. The wilted tissue returns to its 

 original turgid and stretched condition after the concentrated solution has been 

 replaced by water or by a very weak solution. After the second phase of growth has 

 been completed plasmolysis of the cells results in little or no contraction of the tissue; 

 in the third phase of growth the cell walls are stretched but little or not at all. Exten- 

 sibility of the cell wall, under the influence of the pressure of turgor that occurs in the 

 cell, is therefore another internal condition necessary for cell enlargement. 



Many external conditions are also necessary for growth, such as temperatures 

 within certain limits, proper oxygen supply, water supply, etc. 



3. Apparatus for Studying Plant Enlargement. — The enlargement of an organ or 

 plant may be measured by various devices, from the simple metric scale to very 

 precise auxanometers, auxographs, etc. Equally spaced marks may be made upon 

 the surface of an enlarging organ and subsequent determination of the distances 

 between the marks indicates the relative rates of enlargement of the various regions of 

 the organ. 



