Vol. XXIII. N0.4.] 



POPULAE SOIEI^CE NEWS. 



55 



^onje, Pai'Di, ami Garden. 



WHAT A LEAF DOES. 



Whenever a pound of carbon — whether in 

 the shape of coal, wcod, oil, or gas — is 

 burned, three and ^two-thirds pounds of a 

 colorless, inodorous gas, known as carbonic 

 dioxide, or carbonic acid gas, are formed. 

 Although so readily formed by the direct 

 combination of its elements, it is one of the 

 most stable of compounds, and can only be 

 re-separated into its component carbon and 

 oxygen by the most powerful reducing agents, 

 such as heated potassium or sodium. None of 

 the ordinary reducing substances have the 

 slightest eflect upon it, and, if it were not for 

 a most wonderful natural phenomenon, it 

 would remain in the air unaltered forever. 



The growing plant is Nature's laboratory, 

 and the tender, delicate leaf, moved by a 

 breath of air, and withering in the slightest 

 heat, has the power of producing chemical 

 reactions, not only like the decomposition of 

 carbonic dioxide, — which the chemist can do 

 with his more clumsy methods, — but it alto 

 builds up the complex molecules of number- 

 less organic compounds, such as sugar, starch, 

 woody fibre, indigo, madder, quinine, etc., 

 which are of supreme importance to the wel- 

 fare of mankind. 



Carbonic dioxide is always present in the 

 air, to the extent of about .05 per cent, by 

 weight, and this trace of the necessary ele- 

 ment is greedily seized upon by the growing 

 plant, and, by some inscrutable powers, de- 

 composed, and its elements — together with 

 hydrogen and nitrogen — rearranged into the 

 substances named above, with the addition of 

 a few mineral substances obtained from the 

 earth. How the plant does this, we do not 

 know, but we know that it is done by the 

 help of the sun, which sends to the plant, 

 along with its light and heat, radiations of 

 chemical energy, which enable it to accom- 

 plish its stupendous task. 



As the earth cooled from a molten mass, 

 and the first beginnings of life began to 

 appear, we mav assume that about all the 

 carbon then existing was either diffused 

 through the air and water in the form of car- 

 bonic dioxide, or was boiuid up in the crust 

 of the earth in the form of mineral carbon- 

 ates. But, with the first microscopic plant, 

 a change l)egan to take place, and the energy 

 sent from the sun enabled it to decompose the 

 invisible gas, and transform it into woody 

 fibre and other compounds. As the plants 

 died, the greater portion of them decajed, 

 and the carbon in them was oxidized back 

 again into carbonic dioxide. But, under the 

 favorable conditions of the carboniferous era, 

 a part of them were sealed up under the over- 

 lying strata of rock, and slowly transformed 

 into coal, awaiting the coming of man to 

 make use of the carbon formed so many years 



ago by the plant leaves, and to liberate the 

 stored-up energy by again oxidizing the car- 

 bon. Neither did the carbonic dioxide in the 

 crust of the earth remain useless, for, dis- 

 solved by the waters of the primaval oceans, 

 it was separated out by millions of marine 

 animals, the skeletons of which formed im- 

 mense continents, and by which, to this day, 

 the coral islands are being built up. 



We thus see how dependent we are upon 

 the chemical power of the little plant or vege- 

 table cell. Not only all our stores of coal, 

 oil, gas, and wood are formed by them, but 

 all our food as well. All animals are depend- 

 ent, either directly or indirectly, upon plants 

 for their means of subsistence, and if this 

 wonderful power of decomposing carbonic 

 dioxide should be suddenly taken away from 

 them, in a few years all animal, as well as all 

 vegetable life, would vanish from the earth. 



[Original in The Pojmlar Science N io».] 

 TREES IN WINTER. 



BY JANE H. NEWELL. 



Is northern America we rejoice in a yearly spec- 

 tacle which exceeds in richness and variety of color 

 anv other forest scene in the world. As September 

 advances, the swamp maples and sumachs clothe 

 themselves in flaming red; then the elms, birches, 

 chestnuts, and, later, the beeches, imitate the sun- 

 shine. Lastly, the oaks turn with a variety of rich, 

 deep hues, which are the most beautiful and satisfy- 

 ing of all. The reason of this brilliancy of coloring 

 — so much more striking than the woods of Europe — 

 is not understood, although it is often attributed to 

 the greater dryness of our climate. 



It is a common mistake to suppose that the color- 

 ing of autumn leaves is due to frost. In mild sea- 

 sons, the trees are often completely turned before 

 the thermometer has once sunk below the freezing 

 point. The first change of color is a sure sign that 

 the tree is preparing for the winter, and that a 

 change in the cell-contents of the leaves has begun. 



The process of making food is carried on in these 

 leaf-cells. "They are the factories, where starch, or 

 something very similar, is made." The raw mate- 

 rial brought from the ground is here changed into 

 food,- on which plants and animals can live. 

 Throughout the summer, food has been constantly 

 made and carried from the leaves to other parts of 

 the plant, where it has been used for food, or stored 

 as a reserve for the future. When this activity 

 ceases, and the leaves fall from the tree, it would be 

 be a great waste of valuble material if all the food 

 contained in their cells were to be lost. Nature per- 

 mHs no such waste. In autumn, when the life of 

 the leaf is nearly at an end, its food materials are 

 withdrawn and deposited in the stem and branches, 

 tor use in the following spring. This withdrawal is 

 preceded by the breaking up of the contents of the 

 cells. The chlorophyll— the green coloring matter 

 of the leaves — is decomposed, and the products of 

 this change, together with the starch and other food 

 materials, are taken into the interior of the tree. A 

 little yellow or red coloring matter is left behind, and 

 it is this which gives the bright hues to the leaves. 

 When they finally fall, they are mere dead husks, 

 emptied of nourishment, and of no further use to 

 the plant. 



When the leaf falls, a scar is left behind. In 

 plants with compound leaves, like the horse-chestnut 

 or the ash, there is a similar scar at the base of 

 each leaflet. The process which causes the fall 



is a curious and interesting one, and it begins 

 very early in the life of the leaf At the base of each 

 leaf-stalk a layer of cells is formed, which gradually 

 cuts across the whole petiole, and finally completely 

 separates the leaf from its stalk, so that a gust of 

 wind, or the mere weight of the leaf, is sufficient to 

 cause it to fall. The walls of the cells of this sepa- 

 rating layer generally become thickened and water- 

 proof before the leaf falls, so that the scar is already 

 healed. 



This process, then, is one that is a part of the life- 

 history of the leaf, and is not caused by changes in 

 temperature. In climates where the plants are ac- 

 tive during the entire year, the leaves fall gradually. 

 As new leaves are formed, the old are dispensed 

 with, and there is never a time when the plants are 

 leafless. But in our climate, most of the trees and 

 shrubs are leafless for a large portion of the year. 

 This is a provision which enables the plants to live 

 through a long period of cold. By the loss of their 

 leaves, and the withdrawal into safe places of all 

 their food materials, the plants are able to survive 

 uninjured. The fall of the leaves is hastened, al- 

 though not caused, by the cold. We do not under- 

 stand exactly why the leaves all fall at once; we can 

 only say that in our climate, probably those plants 

 have survived and increased which were so consti- 

 tuted that a long period of rest succeeded a season of 

 active work. 



In countries where a wet season is followed by a 

 hot, dry season, the plants lose their leaves when the 

 heat begins. Thus excessive cold and excessive heat 

 produce the same effect. 



Another advantage to the plants in losing their 

 leaves, is the lessened resistance which the tree pre- 

 sents to storms, and especially to snow. The 

 weight of the winter snows would break the trees if 

 the leases were obliged . to carry such a load. We 

 sometimes see trees badly injured in this way by a 

 premature snow-storm. 



Now that the trees are divested of their summer 

 dress, we see that the provision for the next season's 

 garment has been already made. There are the buds 

 thickly studding the branches. Look at the strong 

 horse-chestnut buds, with their resinous, water- 

 proof covering, and think of the wonderful sleep of 

 the leaves within, wrapped in their woolen blankets, 

 and awaiting only the warmth and moisture of 

 spring to burst into renewed vigor. After one 

 has studied naked branches, the trees become 

 as beautiful and as distinctive in their winter cloth- 

 ing as when they are leafy and green. 



If we examine a branch early in the summer, we 

 shall find that the growth of the next season's buds 

 has already begun. At the ends of the branches, 

 and in the leaf-axils, the new buds are forming, to 

 be completed and covered with some protective en- 

 velope in the fall. Through the bitter winter they 

 remain thus, safely covered from wet, and protected 

 from the changes of the season. 



Many leaves in the bud are invested completely in 

 a garment of wool or down, which is a non-conduc- 

 tor, and saves them from being exposed to sudden 

 changes. Young horse-chestnut leaves are thus 

 densely clothed with wool, and the young leaves of 

 the beech are covered with silky hairs. Sometimes 

 the bud-scales are lined with down, as are the inner 

 scales of the red maple. 



The buds are usually covered by scales, which 

 consist of leaves, stipules, or flower-stalks, modified 

 for the purpose of protection. In the lilacs, the 

 scales pass so gradually into leaves that it is hard to 

 draw any distinction between them. The scales 

 of the elm, the beech, the tulip-tree, and mag- 

 nolia, are stipules, and those of the horse- 

 chestnut are modified leaf-stalks. In all these 

 cases, the outer scales have become thickened and 



