Lighter knots are odd growths found in evergreens. For unknown reasons, all trees may lay down wood winding helically around the main axis of a trunk or branch. In woodworking terms, the wood is called 'spiral grained' and it is useless as lumber. As the wood dries, spiral grain will shrink as much as 30 percent and it will not all shrink in the same direction.
A morphogenetic explanation of spiral grain is "the arrangement and orientation of cambial cells very largely predetermines the relative position and arrangement of the various xylem derivatives, and hence the grain angles in wood". (Wood Quality, Impact on Product Yields, Grades, and Values, BC Forestry Continuing Studies Network, no date) Simply put, the first cells started out in a direction different than aligned with the tree's axis and all other cells that grew from those first cells continued growing in that direction.
But sometimes, the direction of the helix changes and sometimes, the direction changes several times creating "interlocked grain". No one knows why wood will grow at odd angles, but theories abound--wind, Coriolis effect, animal damage, and genetics. All theories have been disproved.
Lighter knots are areas of helical grain around branches in southern pines. Often the wood immediately preceding and following the spiral grain appears normal. The spiral grain is heavier than normal pinewood, but this may be due to the tissue being filled with resin or may be the reason the tissue fills with resin.
Lighter knots catch fire easily and burn brightly for a long period of time. Special equipment housed burning lighter knots. Iron boxes and iron baskets attached to paddle wheelers held the burning knots. The knots lit the way--a box sat on top of the pilothouse to serve as a headlamp, the baskets along the sides of the boat lit the riverbanks.
Lighter knots were also used in camp fires, especially before the twentieth century. The bright light lasted the entire night keeping wild animals away. John Finlay (1941-1991) recalls those days in a poem called "Salt from the Winter Sea".
My great-grandmother said each year the men
Would travel to the Gulf--a hundred miles--
To draw salt from the winter sea.
They camped
Beside their wagons in the wild sea-oats
And built up fires of lighter knots and pine...
The Florida State Legislature has a painting depicting how lighter knots were used aboard boats. To view the painting by Christopher M. Still, click on the link:
http://www.leg.state.fl.us/publications/2000/house/special/chamber_art/oklawaha1.pdf 
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