Why do most living lower vascular plants have little or no secondary growth?

Most living lower vascular plants do not exhibit secondary growth because their development is primarily restricted to activity at the stem and root apices. In these plants, once the young organs reach maturity, there is generally no further increase in diameter.1

While some extinct groups of lower vascular plants possessed a cambium capable of producing secondary vascular tissues—resulting in woody growth—this trait is largely absent in extant species. Among living lower vascular plants, only two genera, Botrychium and Isoetes, display even a slight vestige of secondary growth.1

Even in large species such as tree ferns, which can reach heights of up to 25 meters, the plant tissues are entirely the result of primary growth from the stem apex. These plants achieve structural strength not through woody secondary thickening, but through strengthening tissues surrounding their vascular bundles or, in some cases, by developing a mantle of roots.1