By the time this issue reaches most of you fall will have definitely set in, and the odor of burning leaves will be filling the air. Many a youngster will be applying the family rake on the front lawn. As one who has raked his share of leaves in times past and who has often wondered concerning the why and the wherefore of this "curse" of falling leaves upon boyhood at a time when the outdoors were at their best, the reason for annual leaf fall and the mechanics of that feature have always been of interest.
The fall of leaves, which characterizes many of our familiar trees, is of course prompted by the necessity of the rigors of winter. The leaves are organs of "breathing" and excessive transpiration would result during the winter months if these trees did not lose their leaves. This would, of course, result in the death of the tree. So leaf fall is simply one of the preparatory means for the dormant winter period. But such an apparantly simple thing is not without elaborate prepartions.
By the middle of the summer most trees have reached their maximum seasonal growth. The balance of the summer is devoted largely to the maturing of the tissues formed. It is at that time, when most of us are enjoying the warm summer sunshine in the mountains or at the seashore that the tree begins to prepare for winter. At the base of the stem (petiole) of each leaf a layer of special cells are formed. This layer of cells serves to insolate the leaf from the branch -- with the exception of the vascular bundles by means of which plant foods, etc. are transported to and from the leaf. This is the last connecting link between the tree and the leaf. When the layer of loose cells is completely formed two secondary layers of corky tissue are also formed -- one on each side of the original isolating layer. Then the process simmers down simply to a matter of waiting for the proper time and conditions. This arrives with the coming of frost which freezes the moisture in the plant tissue of the petiole and brings about the formation of small ice crystals in the cells of the isolation layers formed in late summer. As the ice crystals are formed they exert a prying action upon the stem or piticle of the leaf, eventually resulting in its being broken from the branch at that point and its fall to the ground.
Such a process would ordinarily expose an open wound were it not for the formation of the corky layers which now act as a sort of protecting tissue. So, in fact, nature provides for the healing of the wound several weeks before it is actually formed!
This pre-healed wound is known as the leaf scar. Examine one with a hand lens or magnifying glass and note the "dots" that appear upon it. These dots are the ends of the vascular bundles through which once flowed the nourishing juices from the leaf to other parts of the tree. You will also be interested to know that trees may be identified during the winter quite readily by the character and position of the leaf scars, together with other accompanying features, for as trees vary in their summer characteristics so they vary in the winter earmarks. There are but few deciduous trees native to Mount Rainier National Park. Our forests are almost wholly coniferous and evergreen. Yet, perhaps a later issue of Nature Notes will point out a few of the differences between some of our common deciduous trees in winter.
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