This was a comment on another thread. Rather than drag it off topic this is a little of what I've learned regarding sappy wood over the years
the rounds were miserable to work with - layer just under bark (cambium?) was a mess. Wasn't until you got into the core that things were pleasant.
The outer wood layer 10-15 rings deep is the sapwood. Technically the cambium is a very thin sheath covering the tree, dividing inward to form xylem, wood, and outward to form phloem the inner bark (what most people mistakenly call the cambium). The outer xylem, the sapwood, is where the sap moves upward from the roots. The phloem or inner bark is where the sugar rich sap flows back down for storage. That is the vertical transport system. The rays form the horizontal transport to feed the cambium and to access the edge of the deeper core for storage
Rays also store carbohydrates, starch. Starch is a comlex molecule formed of simple sugars.
The column of water in the xylem, from the roots to the leaves is in tension. As a molecule leaves the stoma in the leaf a molecule enters the root. The breaking of this tensile column causes an air embolism that generally signals the beginning of the end for the cells in that column and the beginning of their transformation into heartwood. As the water levels drop in the heartwood, starch can hydrolize to sugars which both while decomposing consume oxygen and produce CO2 creating pressure which further destroys the tension based water column. The byproducts of this decomposition are polyphenols, waxes, fats, tannins, resins, gums, oils, aromatic and coloring agents... the spent byproducts of respiration, the extractives.
Sap does not go down in winter, there is no pump to re-establish the water column. Flow stops in hardwoods and it seals itself through the dormant period but the moisture content remains at least the same and sometimes slightly higher. Conifers never stop, if they are not frozen they are growing albeit slowly. The needle is an adaptation for this purpose, narrow and covered by a waxy cuticle to prevent winter dessication. Freezing in hardwood causes an embolism in the water column and it cannot normally be re-established. This freeze line shows itself by species as you drive north. Conifers can bleed freezing sapwater into surrounding interstitial spaces and then reabsorb it into the cell lumens and can re-establish water column, to a point.
Resin is not sap, although we use the terms interchangeably. Resin is produced in specialized cells, epithelial cells, that lie in the interstitial spaces between the normal tracheid cells of most conifers. Resin is believed to be for helping to protect damaged tissue and to help repel invaders. The epithelium consists of longitudinal and ray cells so it has the ability to transport vertically and horizontally. Remember the heartwood is dead and blocked by extractives so there is effectively no transport there. The epithelial cells are the last to die, producing resin for up to 6 months after a tree is felled if left to naturally dry. Resin can also "flow" on any warm day in an air dried timber. I've opened up a fresh face on 160 year old pine, left it in the sun and had sticky sap bead up. Cold resin in the winter is much slower to exude and less sticky than warm sap being pumped by the epithelium in the summer. If you look at a end grain cookie that has bled count the rings of sappy wood before you get to the heartwood. This is telling you how long the average cell lives.
The gathering and processing of resin was a cottage industry in the old days. Longleaf pine was one frequently tapped tree of the early forest. In the 1830 census several of my ancestors reported "turpentine" as their occupation. In '65 Sherman, running behind schedule, illuminated his pathway into NC by lighting that pine forest and marching by its torchlight.
Micrograph of a resin duct;
http://www.botgard.ucla.edu/html/botanytextbooks/generalbotany/barkfeatures/a1438tx.htmlSome Wiki definitions;
http://en.wikipedia.org/wiki/Polyphenolhttp://en.wikipedia.org/wiki/Starch
