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March 21st, 2019

some tree stuff

Currently reading The Secret Life of Trees by Colin Tudge, not to be confused with the more recent The Hidden Life of Trees by a German forester.

What is a tree? The obvious popular, and functional, definition is a tall plant on a stick, outgrowing competition in a race for sunlight. The least interesting definition requires the stick to be made of wood[1], rather than herbal stems kept up by water pressure; I'll call that "woody tree". A more evolutionary definition of "trees proper" invokes secondary growth, and specifically the cambium, a sheath of cells around the trunk that generate wood on the inside and bark on the outside (xylem and phloem), contributing to growth outward as well as upward.

The tree lifestyle is one of the targets of convergent evolution, hit by Lepidodendron, tree ferns, some Carboniferous horsetails, various monocots.

The tree proper encompasses conifers (and their gymnosperm relatives, cycads and ginkgo) and most flowering (angiosperm) trees, which suggests their common ancestor was a tree, and also that the first flowering plant was a tree, despite the vast mass of angiosperms that have since shed all wood and tree-ness.

Flowering plants can be divided into primitive dicots, true dicots (eudicots), and monocots. The big distinction is that monocot leaves grow from the base, rather than the tip or edge; grasses are monocots, and having their growth region below the ground means they can survive grazing, which is part of why they've become so successful in the last 40 million years. There are five groups of monocot 'trees', none of which have the cambium of trees proper, so the first monocot must have been herby, with subsequent re-inventions of the tree lifestyle. Some of those have a form of secondary growth but not the cambium. Monocot trees include Joshua trees and palms.

[1] Lignified cellulose. Cellulose is floppy, having lots of lignin molecules in it makes for a rigid matrix that can stand up on its own.

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