556 GROWTH, ASSIMILATION, AND ACCUMULATION 



all enlarge symmetrically along all axes. Hence cells of very diverse sizes 

 and configurations arise in plant tissues. 



The enlargement of plant cells involves primarily an increase — often many 

 fold — in the volume of the vacuoles, and an areal extension of the cell walls. 

 Some increase in the thickness of the cell walls also often occurs during this 

 phase of growth. During the enlargement phase of growth the cell sap dis- 

 appears from the intercellular spaces and they become filled with air. 



During this phase of growth there is apparently little or no increase in 

 the quantity of protoplasm within the cell. As the cells increase in size there 

 is an influx of water into the enlarging vacuoles. Water is also utilized in 

 the hydration of the additional cell wall material which is constructed during 

 cell enlargement. Relatively large quantities of water therefore become in- 

 tegral parts of the cell system during this phase of growth. As the cell 

 increases in size the cytoplasm gradually becomes attenuated into a thin layer 

 which lines the inside of the cell wall, against which it is held by the pressure 

 of the water in the vacuole. 



Assimilation of carbohydrate foods continues during cell enlargement be- 

 cause of the continued extension and thickening of the cell walls which occurs 

 during this phase of growth. Cellulose and pectic compounds are the prin- 

 cipal cell wall constituents synthesized from carbohydrates. Since little or 

 no additional protoplasm is built up during this phase of growth the con- 

 sumption of proteinaceous foods during cell enlargement is usually small. It 

 is noteworthy that dividing cells assimilate both carbohydrate and proteinaceous 

 foods, while enlarging cells assimilate almost solely carbohydrates. 



The zone of cell enlargement is a region of relatively high respiratory 

 activity. Cell for cell the rate of respiration in regions of cell enlargement 

 is probably little if any less than in meristematic regions. 



Rapid translocation of both water and solutes is continuously in progress 

 towards any region of enlarging cells. Like dividing cells, enlarging cells 

 may possess the capacity of accelerating the translocation of solutes into them 

 as a result of the expenditure of energy. 



Recent investigations indicate that the areal extension of plant cell walls 

 can occur only in the presence of hormone-like substances called "auxins 

 (Chap. XXXII). 



Two main views regarding the mechanism of cell enlargement have been 

 advanced. One of these holds that the cell wall must first be subjected 

 to elastic (reversible) or plastic (irreversible) stretching as a result of a 

 turgor pressure developed by the cell sap. While in the stretched condition 

 it is assumed that the material substance of the wall is increased either by 

 the intercalation of additional molecules in the wall {intussusception), or by 



