Bark or Dermal Tissue
Dermal tissues generally occupy the "skin" layer of all plant organs including trees. These bark tissues are responsible for controlling environmental conditions that continually interact and effect the tree's outside. The dermal tissues influence light passage, regulate gas exchange, recognize and defend against pathogens, control tree temperature and many other things.
Vascular Tissue
The vascular tissues of higher plants (Kingdom Plantae) are divided into two sections: xylem and phloem.
Xylem is basically made of non-conductive, non-growing sclerenchyma support cells (wood) but can contain some conductive parenchyma cells. Xylem is responsible for the transport of water and soluble mineral nutrients from the roots throughout the plant.
Phloem is the living tissue that carries organic nutrients (from photosynthesis) and particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark.
Sandwiched between the xylem and phloem is the very important vascular cambium. The cambium is the embryonic cell (meristem) source for both the secondary xylem (inwards, towards the pith) and the secondary phloem (outwards) and is located between these tissues in the stem and root.
Root or Ground Tissue
Actually, this tissue occupies the space between the dermal tissues and the vascular tissues. These cells are much more than just filler, though. In roots the ground tissue may store sugars or starches to fuel the spring sap flow. In leaves, the ground tissue is that layer doing photosythesis, the mesophyll.
