From Cell to Organ-4 

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 their margins and thick in the middle. This is similar to certain Green Algae (Coleochaete). However, this shape has a relatively large Surface / Volume ratio. 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 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 and   produced upright, aerial, photosynthetic stems.

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