Formation of the shoot system
A terminal bud has a growth cone at its tip: its thin-walled cells divide all the time, the number of cells grows and the stem lengthens. If the tip of a stem is pinched off, lengthwise growth stops, many lateral shoots grow from lateral buds and the plant branches; in gardening this is used to shape trees and obtain larger harvests. Growth speed depends on the species: willow, poplar and plane grow fast; pear, hawthorn and pistachio slowly; a grapevine grows several metres in one summer. The stem thickens through the cambium. In spring much water and food reach the cambium and its cells are large and light-coloured; towards the end of summer the cells are smaller, and in autumn division stops. So each year one annual ring (a wide spring part and a narrow late-summer part) forms in the wood. Counting the rings on a cut surface gives a tree’s approximate age.
“Ring counting”: the teacher shows a photo of a tree cross-section (or a sawn log). Groups count the rings to find the age and try to spot dry or rainy years (thin rings). Safety: beware of rough edges and splinters on a sawn log.