Question

Many fossil lyocphytes were tree-sized plants. It is hypothesized that seed plants now outnumber lycophytes because of the adaptive advantage of the seed, but there may also be an adaptive advantage to novel vascular tissue arrangement. What might be advantageous about the arrangement of vascular tissues in the dicot stems compared to the ancestral condition of lyocphytes? Defend your answer.

Answer 1

Lycophytes are pteridophytes. They are the first land plants that contain vascular tissues. They show huge variation in the vascular tissue arrangement.

Ex: L. serratum = Actinostele

L. Phlegmaria = Polyarch actinostele

L. Clavatm = Plectostele

L. cernuum = Mixed protostele

The vascular tissue arrangement shows huge variation. The major factor that contributed to the extinction of many of the lycophytes is that they do not have cambium in the vascular tissue. So, they do not exhibit secondary growth.

The ring-shaped arrangement of vascular bundles and the presence of cambium in dicots allows them to exhibit secondary growth. Secondary growth increases the lateral size of the stem and supports the growth of plants to high elevation.

Evolution of xylem vessels and phloem vessels also is an adaptive feature seen in higher plants.