200 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



invariably, on the orientation of existing material) and, within a tissue, 

 on surrounding conditions which may impose limits on expansion in 

 some direction. 



Such conceptions are naturally of value only insofar as they will 

 stand up to rigid comparison with data obtained by rigidly controlled 

 experimental treatment of growing cells. An attempt to make such a 

 test is well outside the scope of this book but we may perhaps refer 

 very briefly to perhaps the most interesting growth study which has yet 

 appeared. Working with root segments in which growth occurred by 

 vacuolation only, and therefore free from internal control either by 

 neighbouring dividing cells or by the exigencies of a shoot system, 

 Brown and Sutchffe(72) have established the following facts. The 

 segments will almost double their length when placed in water alone, in 

 a period of some twelve hours. In sugar solutions, however, the total 

 elongation obtainable is much greater, reaching a length about four times 

 the original in a period of about 48 hours. This greater length is achieved, 

 not by an increase in the rate of growth but in the time period during 

 which growth proceeds. If potassium ions are added to the sugar 

 solutions, then still greater extensions are achieved, but now the effect 

 is on rate of growth. This effect of potassium is traced partly to an 

 effect on sugar uptake but, and with higher significance, to an effect 

 on respiration. Cellular respiration can therefore provisionally be 

 associated with rates of growth. In terms of the suggestions made here, 

 this would be understandable as an effect of respiration on the breaking 

 of bonds in the structural components in the wall, and it may be 

 significant that cessation of growth in the water is associated with a 

 depression of the rate of respiration. The influence of sugar would then 

 be expected to be largely on the period during which growth occurred, 

 since growth could proceed only so long as new material could be 

 intercalated. This experimental separation of rates from periods of 

 growth is therefore not out of harmony with our general ideas deiived 

 from structural considerations. 



Here, however, we are going well beyond the bounds of existing 

 knowledge. We have come a long way from our original considerations 

 of the dead cell walls which formed the starting point of our study. 

 From our approach to the study of the static structure of mature cell 

 walls we have found ourselves led into a dynamic study of protoplasmic 

 activity. These are the lines upon which future research will develop, 

 and if the attitude adopted in these last pages errs perhaps on the side 

 of insecure speculation it is nevertheless through self-indulgence of this 

 kind that advances are made. 



