From Cell to Organ-3  

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.

Phloem is specialized for sucrose transport

The next step could have been the multiplication of Vascular Bundles and their interconnection via Lateral Bundles.

This level of complexity is the equivalent of a Leaf! The leaf is an Organ. Consequently, our theoretical plant has reached the Organ level of complexity.

All Plant Organs are composed of Three Tissues (Epidermis, Parenchyma & Vascular)!

The next steps in evolution involved the formation of separate organs which are specialized for water acquisition & anchorage [Root], photosynthesis [Leaf] and vertical growth [Stem].