Wood

The activity of the vascular cambium is not uniform through the year — it responds to the seasons, and this seasonal rhythm is recorded permanently in the wood the cambium produces.

During a favourable growing season, the cambium produces spring wood (also called early wood): the xylem bands it lays down are comparatively broad and pale in colour, the tracheids have thin walls and wide lumens (ideal for rapid water conduction when the plant is actively growing), and fibres are relatively few, giving the wood a low density overall. When conditions turn unfavourable, the same cambium switches to producing autumn wood (or late wood): the xylem band it now lays down is narrower and darker, the tracheid lumens are narrow with thick walls, and fibres are abundant, giving this wood a much higher density. One complete cycle of spring wood followed by autumn wood, laid down over a single growing season, is visible in a transverse section as one annual growth ring — which is why growth rings are prominent in plants that experience a genuinely seasonal climate, and much less distinct where growing conditions stay fairly uniform through the year. …