## 5 Conclusions  
Correlating plants' growth characteristics with the soil contamination levels using statistical modeling can obtain interpolated variations of the mass increase functions to determine several functions that express the time dependence of the heavy metal concentration in the lettuce plants. Starting from these interpolation relations, one can calculate other characteristics of the evolution process for various plants that are growing on contaminated soils. We identified a research tool that describes how different levels of zinc contamination affects lettuce crops, analyzing three growth characteristics: plant mass, plant height, and plant diameter. We used the least squares method on the experimental data, in order to determine the 1-4 grades mathematical regression functions that describe the three growth characteristics in time. The three plant growth characteristics (mass, height, and diameter) have been correlated with the three zinc concentrations in the soil in order to draw conclusions about the effect associated to each concentration on the lettuce experimental culture. The main conclusion of the statistical model, that was validated by experimental determinations, is the appearance of the dwarfism effect. Therefore, as the concentration of zinc in the soil increases, lettuce plants tend to have a smaller size and mass. The functions that describe the mass, height, and diameter, and variation over time, have been extended for two variables: time and zinc concentration. In this way, a better prediction was obtained regarding the plant development, under the influence of zinc concentration in the soil. All these functions can then be used for forecasting, using extrapolation. The techniques that have been used are not necessarily new; however, they are very useful for forecasting and for studying the biodynamics of plant development. The results obtained by the interpolation of the experimental data and their interpretation are likely to give very important indications for the following experimental step: the prolongation of the experiments over at least two to three generations of plants, which will allow drawing some conclusions on the transmission of heavy metals between generations of plants, or, probably depending on other influential parameters of the process, the decrease of the heavy metal concentration in the plants of the following generations. Another important observation is the dependence of the contamination level on the type of soil used. Zinc contamination tests should target several soil types, given the different way in which soil structure can affect the time behavior of pollutants. The paper addressed a specific situation where the contamination was achieved by applying a fixed dose of zinc, while the synergistic effects with other substances were not evaluated. It should be noted that the contamination process found in nature may be different: the contamination might be either continuous for long periods of time, or intermittent with maximum and minimum values. Our experiments will continue the study with the evaluation of plant development for several other soil types, with the analysis of the synergistic effects between pollutants, and with the variation of the concentrations used for soil contamination. Conceptualization, P.C. and M.A.P. ; methodology, P.C., M.A.P., F.N., V.V. and N.U. ; software, P.C. and M.A.P. ; validation, P.C., F.N. and N.U. ; formal analysis, I.G. and O.D.C. ; investigation, V.V. and D.C.; resources, N.U. and V.V. ; data curation, V.V. ; writing--original draft preparation, P.C. and M.A.P. ; writing--review and editing, N.U. and F.N. ; visualization, V.V. and N.U. ; supervision, V.V. All authors are main authors and have contributed evenly to this paper. All authors have read and agreed to the published version of the manuscript. This paper was financed by the support of the Executive Agency for Higher Education, Research, Development and Innovation Funding, Exploratory Research Program, PN-III-P4-ID-PCE-2016-0860, contr.