Final answer:
The third adaptation that allowed for the development of large trees by the Late Devonian was secondary growth, which lets plants increase in thickness and support larger biomass. This development led to the first forests and influenced the evolution of terrestrial ecosystems.
Step-by-step explanation:
By the Late Devonian period, plants had undergone several adaptations that permitted them to grow large and thus gave rise to the development of large trees. The initial adaptations included the development of vascular tissue and well-defined leaves and root systems. However, the third critical adaptation that allowed plants to grow tall and become large trees was secondary growth.
Secondary growth is the process that enables plants to increase in thickness or girth as a result of the formation of new tissues by the cambium, a layer of undifferentiated cells. The presence of secondary growth allows plants to support greater biomass, thereby contributing to the development of extensive forests with large trees. These large plants began to dominate the landscape by the end of the Devonian, leading to the first forests that are depicted in the fossil record.
This ability for secondary growth facilitated the evolutionary pathway toward the towering forests of the Carboniferous Period, populated by club mosses, horsetails, ferns, and the primitive seed plants that formed these vast ancient woodlands. These adaptations were essential for plants to successfully establish themselves on land and form complex ecosystems, which influenced the subsequent evolution of terrestrial animals, including the early vertebrates that began to inhabit these wooded landscapes.