SUMMARY.In this chapter we have continued our analysis of the results of the experiments on the growth of maize carried out by Kreusler and his co‐workers. The rate of growth has been expressed per unit leaf‐area instead of per unit dry‐weight as in the last chapter. The term “Unit Leaf Rate” is used for the weekly rate of increase of dry‐weight in mgs. per sq. cm. The Unit Leaf Rate, instead of undergoing a perfectly definite type of variation, as does the Relative Growth Rate, fluctuates about a mean value. The larger fluctuations which occur in the values for Unit Leaf Rate calculated for the later phases of the life‐cycle have been attributed mainly to sampling errors.Correlations between Unit Leaf Rate and various environmental factors have been determined.The general evidence is that the Unit Leaf Rate is correlated more closely with temperature than with any of the other environmental factors.By allowing for respiration on the basis of our own experimental results values for the real assimilation were arrived at. These also show a closer correlation with temperature than with light.The values for assimilation determined from the Unit Leaf Rate are of a lower order than those determined by the “half‐leaf” method, but much higher than those determined by the “gasometric” method.Finally, the authors wish to express their indebtedness to Dr F. F. Blackmail for his stimulating criticism and help in this and