Thallus Shape     

Thallus Shape is important because plants need to minimize water evaporation. A sphere has the least amount of Surface Area / Volume ratio.

A flat sheet has the greatest Surface Area / Volume ratio.

Water would evaporate more readily from the sheet because more molecules are in direct contact with the atmosphere.

However, plants need to perform Photosynthesis in order to survive. Consequently, plants need to maximize their surface area to intercept as much light as possible. The process of evolution has selected plants which are best able to balance these two conflicting needs.

The first land plants probably resembled a discus which was thin at the margins and thick in the middle. This is similar to certain Green Algae (Coleochaete). However, this shape has a relatively large Surface / Volume ratio.

Some Thallose Liverworts (Anthoceros) have a shape like this but there are no large land plants with this kind of shape. Consequently, other shapes must have had greater adaptive value.

A strap-like thallus would have less surface area compared to a disk-like thallus and there are some land plants like Marchantia that have this kind of shape. This kind of thallus would be multilayered in its center and unilayered at its margin.

A Cylinder has even less Surface Area / Volume than a strap-like structure. The first fossil land plants had cylindrical thalli and some extant plants have retained this shape. These thalli would be similar to stems and roots.

The first cylindrical organisms probably had horizontal photosynthetic stems called Stolons. Specialized, absorptive underground stems called Rhizomes developed later and eventually upright, aerial, photosynthetic stems arose.

All of these had a simple Tissue organization of  with Vascular Tissues in the center, surrounded by Ground Tissue and Epidermis.

Initially, all of these Organs had one central strand of Vascular Tissues (Xylem & Phloem), a cylinder of Photosynthetic Parenchyma (Chlorenchyma) and an Epidermis.

The Psilophyta  illustrate these evolutionary advances.