776 



PLANT GROWTH 



lO 



inherent differences in auxin sensitivity in the different layers, aided and abetted 

 by an inhibition of extension caused by the wound itself. This latter effect is 

 presumably minimal when coleoptiles or other hollow organs are slit, since the 

 area of wound is very small. Besides, tissues wounded on all sides can still curve 

 (Thimann and Schneider, 1938). More detailed discussion cannot be given here, 

 but it is clear that the influence of the elongation of one cell on that of another is 

 of importance in the growth of all kinds of plant tissue. It has the result that the 

 elongation of the cells in an intact root or shoot is held to a certain uniformity. 



From time to time it has been suggested that elongation of an organ includes 

 "sliding growth" or slippage of the apex of one cell past another, as has been seen 

 in the tips of long cambium initials. However, careful observation of the individual 

 cells in the epidermis of growing roots, particularly by Sinnott and Bloch (1941) 

 offered no support for any such independence of action. More recently the idea 

 that cells might be mainly elongating at their tips was revived by Miihlethaler's 

 electron-microscope studies of the cell walls in young coleoptiles (1950). These 

 showed that the wall at both the apical and basal tips of these elongating cells was 

 much thinner than in the middle. Whatever the explanation, the difference in 

 thickness does not appear to be due to tip growth since the application of fine 

 copper oxide particles to the surface of a growing coleoptile shows clearly that 

 at least in the epidermis the elongation is uniformly distributed along the length 

 of each cell (Castle, 1955). 



For analysis of cell enlargement it will be convenient to consider in order the 

 different factors which control the process, and then to try to visualize its essential 

 character. 



{b) Control by auxin 



In shoot tissues, such as elongating stems, young leaves and buds, the auxin 

 level, as determined by diffusion experiments, is of the order of 25-200 [ag/kg 

 fresh weight. Reference to Table 2 (p. 758) shows that this value is towards the 

 low end of the range for cell enlargement; in other words, cell enlargement in 

 growing tissues is commonly limited by the auxin available. This qualitative con- 

 clusion can also be drawn from the numerous instances where modification, 



Concentration of lAA 



Molal 



Fig. 7. Scheme of growth responses to auxin by different tissues. The curve for flowering 

 refers to long-day plants growing under conditions close to the threshold for flower induc- 

 tion. The auxin concentrations are approximate only. From Leopold and Thimann, 1949. 



