“…In the first instance, such experiments allow the comparison of some photosynthetic indices using plant samples collected in different situations. Examples are provided by studies of the variation of photosynthetic capacity with latitude (e.g., 144,212) or season (12,64,127,162,182,257), and of differentiation between "sun" and "shade" material (10,20,207). The appli cation of such measurements to a reconstruction of photosynthetic activity in nature requires more extensive sampling and measurement of environmental variables than is usually made; direct comparisons of laboratory and field exposures (e.g., 220) are rarely attempted.…”