The objective of this thesis was to search for changes in the growth behavior of plants that may have been caused by the dramatic increase in atmospheric CO2 from about 290 ppm in pre-industrial times (before 1850) to actually 370 ppm. As an excellent candidate for this purpose turned out to be Araucaria angustifolia, a tree that forms annual growth rings and exists in big numbers in the State Park in Campos do Jordao, in the State of Sao Paulo, Brasil. To detect any possible change in the rate of radial stem growth over decades needed a new approach to the analysis of the tree ring series. This was the first problem to be solved for this thesis, and the solution was, to consider the relationship between the radius and the number of each ring. This gives a very smooth curve that can well be described by a two parameter function with correlation factors normally in the range of 0,99 less or equal r2 less or equal 0,999. The two parameters, a and b, are derived from the best fit to the measured data pairs [R(N),N], where N is the ring number and R its radius. These two parameters, together with the knowledge of the ring numbers (or at least the number of the outermost ring, i.e. the age of the tree) are describing completely and in a quantitative way the radial growth behavior of an Araucaria. With these numbers, derived from all analysed trees, it was possible to detect and describe in a quantitative way differences in the growth behavior. There was one group six trees available that grew in the 16th, 17th and 18th century. The other group were 28 recent trees, i.e. trees of actually 35-70 years. It was assured that the trees forming each group was the product of special growth conditions. The detected differences are quantitatively described by the value of the parameter b that had in all old trees a value around 0.5 or less, while in the recent trees it is close to 1.0. This difference means: in the old trees, each ring is, on the average, smaller than the one before; while in all recent trees, all rings have, on the average, the same width. This last behavior leads to the conclusion that the radial stem growth of recent trees is superior to that of trees that grew in former times. This was not found in a statistically significant way when comparing the radii of corresponding rings, as e.g. of the 30th, 38th, or 50th ring. The differences are still too small. However, when extrapolating the observed growth curves, the growth rate of recent trees will be superior to that of trees that grew in former centuries considerably. Most interesting was the analysis of trees that germinated around 1850 and are still alive. Their ring series were analysed in intervals of 10 to 10 rings and thus it could be seen that the value of b grew steadily from around 0.5 to around 1.0 during the last 80 years or so. From these results it is evident that the environmental factor that caused this change (whatever it might have been) developed during this period with, as it seems, an increasing intensity. About the nature of this factor we can only speculate. We finally believe, after checking and eliminating other possibilities (like changes in weather conditions or unintended fertilization from industrial emissions) that the change in atmospheric CO2 was, directly or indirectly, responsible.