Paul Tait, of South New Brighton School, asks :-

How does seaweed grow?

Cameron Hay, a marine biologist with the Cawthron Institute in Nelson and who has an interest in developing seaweeds as candidates for new aquaculture species, responded.

Seaweeds get their energy to grow by photosynthesis. From sunlight and a green chemical called chlorophyll, they make the sugars needed for growth. Although chlorophyll often makes plants look green, many seaweeds are brown or red; but they still contain chlorophyll. This can be seen by boiling the seaweeds to remove the red or brown substances, leaving a green plant. Seaweeds absorb water and nutrients directly from seawater. So they don't need roots or complicated internal plumbing. Though some seaweeds appear to have roots (called holdfasts), their purpose is just to grip tightly to the rock or anchor the seaweed in mud.

By not needing roots, some seaweeds drifting about after storms keep on growing. The Sargasso Sea is named after bits of Sargassum seaweed trapped by circling currents in the Atlantic Ocean to form a floating forest. In estuaries, fragments of seaweed can keep on growing as they drift about in the tides, for example sea lettuce.

All plants grow from meristems which are plant cell factories churning out new cells. Meristems of trees and shrubs are mainly near the plant tips. For kelp seaweeds, thrashing about in the waves, the tips of the leaf-like blades are soon worn away, so the meristems are near the base of the plant. Therefore if you punch a little hole in the base of a kelp blade, then the hole travels up the blade to the tip where it is worn away.