HOW PLANTS FIGHT THE COLD.

Auckland Star, Volume LXV, Issue 267, 10 November 1934, Page 7 (Supplement)

HOW PLANTS FIGHT THE COLD.

Winter, with its keen frosta and bitter winds, is a difficult time for all forms of vegetation. Plants, no less than animale, must conserve their vital warmth if they are to continue to live. Even in the summer many plants, such as clover and wood-sorrel, draw their leaflets together in order to guard against loss of moisture during the night. In freezing weather the task of keeping warm is still more formidable. Most of our big trees shed their leaves at the approach of winter and retire into a semi-dormant condition, whilst large numbers of annual plants disappear altogether. Still, there remains much growing vegetation , which has to face the full rigours of the season. When an animal wants to keep warm it huddles itself up, and in a very similar manner plants tight the cold. In frosty weather the leaves of even the hardiest weeds are drawn closely together eo that the surface from which heat can radiate is reduced to a minimum. Even more striking is the appearance of evergreen shrubs, such ae laurels and rhododendrons, which, in their mode of responding to temperature, are almost as reliable as therniometere. In the mild conditions the leaves of the rhododendrons, for instance, are held well above the horizontal. As the temperature sinks each leaf drope, until by the time freezing point is reached it will be standing out at a right angle from the stem. Should the frost increase, the leaf drops lower still, until, with a great degree of cold, the foliage hangs so that the tips are almost pointing to the ground. Moreover, the edges of the leaves tend to curl inwards, and the whole shrub seeme to be drawing itself together in order to keep warm.

Quite apart from the frost there is another danger to plant life during extremely cold weather. This is the excessive evaporation of moisture which takes place. A lump of ice suspended in the open will steadily become smaller through lose of moisture even if the frost does not relax for a single moment. In the same way every part of the plant that is above ground is continually giving up water in the form of vapour. The process is one which is full of menace to the plant, for, owing to the chilled state of the soil, root activity restricted and the lost moisture is not readily made good. During severe weather it is likely that quite as many plants die from an undue loss of moisture as from extreme cold, β-xcessivfi transpiration in evergreens is largely prevented by a hard, glossy layer on the upper (surface of the leaf,, which is so well seen in the case of the holly.