How did secondary growth allow some ancient lycophytes to become trees?

Ancient lycophytes overcame the size limitations of herbaceous plants by evolving the ability to undergo secondary growth, which allowed them to increase their stem diameter and reach tree-like proportions.1

Unlike many modern seed plants that primarily produce wood through a vascular cambium, much of the secondary growth in ancient arborescent lycophytes occurred within their cortical tissues. This process enabled these plants to develop into massive structures, such as the extinct genus Lepidodendron, which could grow up to 40 meters (130 feet) in height.12

By developing this capacity for secondary thickening, these lycophytes were able to support larger bodies and form extensive forests during the Carboniferous Period. While this growth provided the necessary structural support for their height, it differed from the wood-based secondary growth seen in modern trees, which is driven by the vascular cambium producing secondary xylem and phloem.134