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Contact Parenchyma - Contact parenchyma connect axial and radial parenchyma. The cells that have pit connections with the transport cells and with the radial parenchyma are called contact parenchyma. Contact parenchyma, are usually axial parenchyma. More insight on contact parenchyma from MODERN ARBORICULTURE - The radial parenchyma contact the axial parenchyma that are in contact with the vessels. The axial parenchyma that are in contact with both the vessel and radial parenchyma are called contact parenchyma. The contact parenchyma play key roles in the symplast. When contact parenchyma are killed, plugs will not form. In some diffuse porous woods such as sugar maple, Acer saccharum, that is tapped for sap to make syrup. Chemicals are added to the tap hole to prolong the flow of sap. Paraformaldehyde in pill form is often put into the tap holes. The chemical kills the contact parenchyma and the vessels do not become plugged with gums or granular materials. The tap holes produce more sap. The treatment leads to early decay because vessel plugging is a first line defense against vertical spread of pathogens. Heavy use of paraformaldehyde, overtapping, high vacuum tensions, cutting of all trees other than maples in a maple bush, and the great number of roads and machine travel have drastically changed the maple syrup industry. The trees are so weakened that minor stresses now become major ones. Acid rain is blamed. The real pathogens are called greed and ignorance of tree systems!

See "Parenchyma Cells"

For much more see MODERN ARBORICULTURE by Dr. Shigo.