The fossil remains of the earliest known trees from the Upper Devonian and Carboniferous periods are either without growth rings or only have them very weakly developed. Since these trees grew largely within the low palaeolatitudes of those periods, their ring characteristics seem quite consistent with growth in an almost seasonless climate. In more recent geological periods, particularly the Mesozoic, fossil wood with very wide rings is found in very high palaeolatitudes where at the present day tree growth is either very limited or absent altogether. It seems likely that the main climatic difference between the present day and the Mesozoic high latitudes is the low ambient temperature which depresses metabolic processes such as transport and growth rather than photosynthesis. For the latter, the supply of light energy at high latitudes would appear to be adequate to explain the high productivity in the Mesozoic provided there had been higher ambient temperatures.