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 of this type, highly elongated cells with some wall thickenings 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.

These Thalli of Pallavicinia are translucent. and are once cell thick along the margins. They have a ribbon-like shape due to the activity of an Apical Meristem (Apical Cell) at the tip of each Thallus. They also have a thick "Midrib" that contains conducting cells.	The lower surface has hair-like Rhizoids.
This is an SEM image of the under (Ventral) side of a thallose liverwort: Note the location of the Apical Meristem and the Rhizoids.

Pallivcinia Thalli with central Nerves

SEM image from the Nerve of Pallavicinia: The narrow cells in the center are the Hydroids.

The next step in the evolution of land plants was the development of plants with Apical Growth that lead to a Leaf-Stem type of organization & .

This is illustrated by the Leafy Liverworts (Hepatophyta).