WHY DO LEAVES FALL?

Evening Post, Volume CVII, Issue 32, 9 February 1929, Page 20

WHY DO LEAVES FALL?

WHAT HAPPENS

Why tlo leaves fall? Tho answer is still incomplete, for the botanists- have not told us yot how it is that a foistingoff layer grows across tho base of the leaf-stalk and brings about the dislodgcincnt (writes J. Arthur Thompson in a deoply interesting article in the "New Statesman")- We know what happens, but we do not know what physiological stimulus leads •to this new growth at the appropriate time. On" tho other hand, it is useful to recall the fact that the leaves have been working hard throughout the summer months building up sugar and other, carbon compounds out of soil-water and air. For it is likely enough that the furnishings of the leaf laboratories suffer from wear and tear. Moreover,'the supply of water is cheeked by the lack of warmth in earth and air, and the slowing of tho sap-current robs the leaves of their vigour. Unless they are evergreens they soon begin to lose more water than they are gaining, and this is the beginning of the end. Something must also bo allowed for! the usual constitutional adjustment of tho unfolded leaf's life tenure to a few months,'a kind of punctuation that we are. familiar with in other and very, different connections, such as the moult-| ing of feathers. It is useful to the bird to get its feathers, somethimes tattered and torn, replaced by a fresh growth at the end of summer, especially if a migratory flight is soon to follow. By a selection of variations in constitutional rhythms an annual moult has been established in most birds, and an annual fall of the leaves in most trees. The advantage of this punctuation to the tree is that it reduces the surface of vulnerability during the winter, and also reduced the demands for water. In evergreens the skin is usually toughened and the transpiration of water-vapour into the air, is less than usual. But even in evergreens the leaves have a limited length of life, the maximum being about ten years, the average much less. '.. , :'

What happons before the leaves fall? Besides the change of colour, to which we shall return, thero is in the dying foliage ah interesting retreat of most of the useful material into the stem. There is a transference of living mat-

ter and its products into the permanent parts of the plant, so that the falling leaves contain little but dead skeleton and waste products. Here' we have one of tho few occasions when plants can be said to get rid of nitrogenous waste as animals do, for although plants form nitrogenous excretions, even urea in somo cases, they are ablo by a neat physiological arrangement to mask the poisonousness of these, and to use them over again' as a source of nitrogen. This is very important since plants are restricted to the nitrogen supplies that are available in tho nitrates and so forth of tho soil. Whilo animals are apt .to suffer from, excess of nitrogen, plants are more likely to suffer from a doliciency; and in the sundews and but-ter-worts on the moor we sco that in-sect-catching has been resorted to as a way of making up for the scarcity of jiitrogenous salts in the unready soil. But our particular point'here is that the shedding of the leaves may be said to includo an actual excretion of waste. Was there over a finer instance of "beauty for asTies'"?