MAPLE FAMILY 



substances which the plant must needs discard are in tlie form ot nearly insolu- 

 able crystals, and by remaining in position mi the leaf drop with it to the ground. 



•The plastic substances within the leaf which would be a loss to the plant if 

 thrown away undergo quite a different series of changes. These substances are 

 in the extremest parts of the leaf, and to pass into the plant body must penetrate 

 many hundreds of membranes of diffusion into the long conducting cells around 

 the ribs or nerves, and then down into the twigs or stems. The successful 

 retreat of this great mass of valuable matter is not a simple problem. 'I hese 

 substances contain nitrogen as a part of their compounds, and as a consequence 

 are very readily broken down when exposed to the sunlight. In the living 

 normal leaf the green color forms a most eftecti\'e shield fro;n the action ot ihe 

 sun, but when the retreat is begun, one of the first steps results in the disinte- 

 gration of the chlorophyll. This would allow the fierce rays of the S ptember 

 sun to strike directly through the broad expanse of the leaf, destroying all 

 within were not other means pro\'ided for protection. In the first place, when 

 the chlorophyll breaks do~wn, among the resulting substances formed is cyano- 

 phyll which absorbs the sun's rays in the same general manner as the chloro- 

 phyll. In addition the outer layer of cells of die leaf contains other pigments, 

 some of which have been masked by the chlorophyll and others which are 

 formed as decomposition products, so that the leaf exhibits outwardly a gor- 

 geous panoply of colors in reds, yellows, and bronzes that make up the autumnal 

 display. 



At a time previous to the beginning of the withdrawal of the contents of the 

 leaf or the formation of the autumnal colors, preparations have been steadily in 

 progress for cutting away the leaf when the proper time should arrive. At 

 some point near the base of the leaf-stalk the formation of a layer of special 

 tissue had begun between the woody cylinder in the centre and the thin epi- 

 dermis. When the time for the casting of the leaf arrives, this special tissue 

 grows rapidly, pushing apart or cutting the cells which have held the leaf 

 rigidly in position in such manner that finally the leafstalk at this point consists 

 of the brittle cylinder of wood surrounded by the loosely adherent cells of this 

 newly formed layer of separation. The merest touch or breath of air will split 

 the layer of separation, break the wood, and allow the leaf to fall to the ground, 



— D. T. MacDoug.\l. 



The great leaf fall of the northern states conies some time 

 between the fifteenth and twenty-fifth of October. As has 

 been explained the leaves have virtually parted company 

 with the tree some time before ; they have been falling 

 since the first, and the ground is strewn with them, but as 

 you look at the trees they show no perceptible diininution of 

 foliage. But about the third week of October something 

 happens — it may be a wind or rain storm, a heavy frost, or 

 two or three days of excessively hot weather — and then the 

 leaves come pouring down in showers, and though the oaks 



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