186 CONTROL MECHANISMS IN CELLULAR PROCESSES 



workers have tended to assume probably because promotion of cell 

 enlargement is an action so characteristic of auxins. Galston and 

 Purves rightly feel that the primary action has so far eluded certain 

 detection. The present discussion is concerned with the neverthe- 

 less important question, what is the mechanism by which a plant cell 

 grows, and how does auxin cause it to be promoted, whether the 

 action of auxin be of a "primary" or "secondary" character. 



To introduce this problem we shall first discuss some aspects of 

 present thinking on the mechanism of plant cell growth, though we 

 cannot hope to survey the bulk of literature which lies behind the 

 various views which prevail. Our interest will be restricted to the 

 kind of growth by cell enlargement which auxin promotes strongly, 

 that is in stems and stemlike tissues or organs such as grass coleop- 

 tiles, not roots (nor leaves). Most cells in such situations grow 

 either throughout their surface area, or if "elongating" in one direc- 

 tion, then throughout their length, rather than growing at a local 

 area at the tip as do root hairs, pollen tubes, and several other types of 

 plant cells. Sections of young stems, coleoptiles, and certain other 

 similarly behaving tissues will grow quite considerably if excised 

 from the plant and placed in a suitable simple medium containing 

 auxin; most ideas about the mechanism of plant cell growth are 

 based on experiments with such material. 



Uptake of Water 



Unlike typical growth in animals or microorganisms, plant growth 

 does not consist primarily of protein synthesis. While cell enlarge- 

 ment in the intact plant is normally accompanied by vigorous syn- 

 thesis of protein, this synthesis commonly is far from sufficient to 

 keep pace with the volume increase of the cell. Excised tissues may 

 grow, furthermore, with relatively minor synthesis of protein (see 

 discussion by Thimann and Loos, 1957 ) . The predominant material 

 which is added to the plant cell is simply water, which enters and 

 expands the cell's vacuole and makes the cell larger. The promi- 

 nence of water uptake in the growth of plant cells led to a tendency, 

 in the past decade, to expect that water uptake would be the process 

 which auxin affects. 



Plant cells have, of course, an inherent osmotic ability to absorb 

 water, due to their osmotic potential (Ti), which as the cell takes 

 up water tends to be balanced by the development of turgor pres- 



