(WITH ELEVEN FIGURES) THE structural differences between sun leaves and shade leaves of several species have been described in a classical memoir by E. STAHL.' FR. JoHow has given an excellent summary of the adaptations of foliage leaves with reference to transpiration.2 LEON DUFOUR has investigated many of the differences in the vegetative and the reproductive organs of phanerogams due to differences in the amount of light supplied to them.3 The writer has not at present access to any tolerably complete collection of botanical periodicals, but neither in ALFRED BURGERSTEIN'S bibliography nor in such journals as were accessible has he been able to find mention of any paper which discusses experimentally the subject of transpiration in leaves of the same individual, some developed in the sun and others in the shade. It would seem that the study of the relative activity of sun leaves and shade leaves must give results of value. For such an investigation no leaves can be more suitable than those of such evergreens as the Mediterranean species of what Schimper calls the Hartlaubflora, Olea, Quercus Ilex, Myrtus, and their congeners. For it is evident that leaves which are active during a period of from one to several years, and which during all of that period are respectively exposed to illuminations varying from 2 per cent. to ioo per cent. of the total amount afforded by the sun, may be expected to show far more notable differences in structure