Heartwood and sapwood consist of d. secondary phloem.
In contrast to heartwood, which frequently makes up the majority of the cross-section of a stem, sapwood is the living, outermost part of a woody stem or branch. The paler color of sapwood usually makes it easy to tell apart from heartwood.
However, wood's color can be deceiving; not all heartwood is dark, and not all dark-colored wood is heartwood. Additionally, different people, species, and growing environments might have quite different ratios of sapwood to heartwood in any given stem. So, a more thorough grasp of what wood is and how both sapwood and heartwood form is necessary for a more true - and less flimsy - difference. This is safe to say.
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