From Cell to Organ-4  

In order for a complex colony or organism to survive, water  and sucrose must be distributed throughout the organism. Cells are interconnected by small cytoplasmic channels in their cell walls called Plasmodesmata. Furthermore, small molecules can move by diffusion through the water in the Cell Walls. However, both of these process are extremely slow and could not be sufficient for large organisms to survive. Vascular Tissues are specialized for the transport of water and sucrose dissolved in water. The most simple Vascular plants have one vascular bundle composed of Xylem and Phloem.

Xylem is specialized for Water Transport BUT also supplies Structural Support! This is required for Vertical Growth.

Phloem is specialized for sucrose transport.

Some Thallose Liverworts have a central Nerve (Midrib) that contains cells modified for conduction but they do not approach the levels of differentiation seen with Xylem and Phloem. In the most advanced organisms, highly elongated cells with some wall thickening occupy the center of the Nerve.These resemble Tracheids and are called Hydroids. However, they lack most of the features associated with Tracheary Elements in seed plants.

The next step in the evolution of land plants was the development of plants that had a Leaf-Stem type of organization.

This is illustrated by the Leafy Liverworts (Hepatophyta).