AskNature

“Groundsels also grow here [on Mount Kenya]. They are relatives of the dandelions and ragworts that flourish as small yelllow-flowered weeds in European gardens. On Mount Kenya, they have evolved into giants. One grows into a tree up to thirty feet tall. Each of its branches ends in a dense rosette of large robust leaves. As the branches grow, so each year the lower ring of leaves in the rosette turn yellow and die. But they are not shed. Instead, they remain attached and form a thick lagging around the trunk. This is of crucial importance to the groundsel. The living leaves in the rosette contain special substances that prevent frost damage to the tissues and even though they may become covered by hoar frost during the night, they thaw out rapidly in the powerful warmth of the morning sun. But then the water within them starts to evaporate through their pores. If the liquid in the supply pipes running up through the trunk were to have frozen during the night, then the leaves would now be unable to replace their water and they would be baked dry and killed. The lagging of the dead leaves, however, prevents the pipes within the trunk from freezing and that particular danger is averted…The solution, however, generates another problem — this time a nutritional one. Retaining the dead leaves on the trunk prevents the nutrients in them from being released into the soil where they could be reclaimed by the roots. The giant tree-groundsel overcomes that difficulty in the same way as the giant cushion plant of Tasmania. It sprouts rootlets from the side of the trunk which thrust their way into the lagging and extract what nutriment remains there.” (Attenborough 1995:260)