554 GROWTH, ASSIMILATION, AND ACCUMULATION 



vacuolate, but all such cells which have been adequately studied have been 

 shown to contain minute vacuoles (Zirkle, 1937). 



The cells in the older portions of the meristem differ in certain respects 

 from those of the promeristem. Priestley ( 1929) distinguishes these as "vacuo- 

 lating and dividing cells." Cell divisions continue in this zone of the meristem 

 at the same time that the vacuoles are increasing appreciably in volume. Some 

 increase in the size of the cells occurs during this period in their growth and 

 the walls become distinctly thicker. Intercellular spaces also appear at this 

 stage and are at first filled with a dilute sap. 



The zone of cell division in any actively growing region is a center of 

 intense assimilatory activity. Since every cell formed as a result of mitosis 

 contains its own complement of protoplasm and since every cell division in- 

 volves the formation of a cross wall between the two daughter cells as well 

 as some extension of existing walls, both carbohydrates and proteinaceous foods 

 are assimilated during cell division. Cellulose, pectic compounds, and other 

 cell wall constituents are produced by the condensation of the molecules of 

 simple soluble carbohydrates. Protoplasmic proteins are formed principally 

 by the condensation of amino acids which are probably transported to the 

 meristem as such or which may, at least in some species or under some con- 

 ditions, be synthesized in the meristematic cells from carbohydrates and nitrog- 

 enous compounds. 



In the promeristem, at least, the quantity of protoplasm built up is pro- 

 portionately very large as compared with the quantity of new cell wall ma- 

 terial constructed. Hence assimilation during this phase of growth consists 

 predominantly of the synthesis of proteins. 



In the vacuolating, dividing cells it is probable that proportionately more 

 carboh^'d rates are assimilated than in the cells of the promeristem since during 

 this phase of growth considerably more extension and thickening of the walls 

 occurs than in the promeristem. 



Utilization of water in hydration of protoplasm and cell walls, and to 

 a limited degree in vacuolation, also occurs in meristematic regions. 



Certain mineral elements in addition to those which are constituents of 

 proteins are also required if the activity of meristematic cells is to be main- 

 tained. The more important of these are potassium, iron, boron, manganese, 

 magnesium, and calcium. Several of these elements are believed to act in a 

 regulatory or catalytic capacity in meristematic cells. 



During mitosis, therefore, a constant translocation of water, soluble foods, 

 and certain mineral elements in the form of soluble compounds is in progress 

 towards the region of dividing cells. Since the vascular system terminates 

 some distance below the region of the stem in which cell division is occurring, 



