The research presented in this thesis investigated the functional morphology in root systems in relation to their role in providing anchorage and stability for the plant. The anchorage of different types of root systems was investigated as well as the influence of several environmental factors on their development. The research presented in this study was completed by carrying out a series of modelling, glasshouse and field experiments using physical models and real plants. Model experiments showed that solid shapes like bulbs are very well suited to resist vertical upward forces, i.e. uprooting, and shed some light on the mechanism of anchorage in bulbs. The results of this laboratory study showed that the concept of optimal bulb shape for resisting uprooting is viable. Uprooting tests on real bulb plants confirmed the theoretical predictions about it, and showed the importance of bulbs in anchorage. This study also proved that the soil type is very important when considering the anchorage of solid forms such as the bulbs. A second model study showed that the simplest models of tap root-dominated root systems increase their resistance to overturning with the third and second power of the embedment depth in cohesionless and in cohesive soil respectively. Anchorage strength of a root system dominated by a tap root will be maximised with minimum investment in structural material if the rigid tap root is extended to the largest possible depth. Glasshouse experiments investigated the effects of soil compaction and temperature, two of the most important environmental factors, on the axial and lateral development and growth of the root systems of two species of young pines. It was shown that the rate of root axial development in both investigated species decreased with an increase in soil compaction whereas the lateral proliferation of their roots systems was not significantly affected by soil consistency. A temperature of around 15°C seemed to be optimal for the root elongation rate since the increase in axial length of the roots of both species was largest at this temperature. The effect of mechanical stimulation as a factor in shaping the root systems of plants was also investigated. Apart from the changes caused to the parts of the tree above ground, unidirectional periodical flexing induced an increase in total root CSA and larger biomass allocation to the roots parallel to the plane of flexing which, in turn, resulted in a larger number of major lateral roots with larger CSA in the plane of flexing. Mechanical and morphological field studies on two Pinus species investigated the anchorage of plate root systems and showed that lateral roots in older trees are not the major source of root anchorage in either of the species; although in both species a certain asymmetry in the distribution of major lateral root CSA was recorded, it was not significantly correlated to the asymmetry in anchorage.