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Winter as a Factor in the Xerophily of Certain Evergreen Ericads

1914; University of Chicago Press; Volume: 57; Issue: 6 Linguagem: Inglês

10.1086/331343

1940-1205

Frank C. Gates,

Plant Ecology and Taxonomy Studies

1. The determination of the rate of transpiration per unit area of leaf surface by weighing is a satisfactory approach to a knowledge of the demands of plants for water, one of the most essential features of their environment. This unit was taken as grams per hour per 100 sq. cm. of leaf surface. 2. While the use of rooted plants is most desirable, the results obtained through measuring the transpiration of cuttings, the stomates of which are closed at the time of collecting and during the period of adjustment to the apparatus, approach closely those of potted plants, except under extreme conditions of evaporating power of the air. Experiments run in duplicate are a check upon the behavior of individual specimens. 3. Cuttings of hydrophytic plants wilt in a very short time in spite of every precaution, and experiments of any duration cannot be made upon them. Cuttings of other herbaceous plants, if properly cared for, may be used for at least 24 hours, and cuttings of shrubs and trees may be used for a still longer time. 4. From the data obtained in this investigation, transpiration varies with the evaporating power of the air, that is, increases directly with increase of temperature, decrease of relative humidity, increase of air movement, and, other factors being equal, is greater in daylight than in darkness. The last factor, however, is due to the tendency toward increased internal temperature with absorption of radiant energy. 5. The transpiration of all plants experimented with was very low in winter, yet was demonstrable during the daytime of clear days, even at the lowest temperatures (-29⚬). Very frequently no transpiration could be detected during the night. The decided gain in weight which frequently occurred is to be attributed to the power possessed by the leaves of these evergreen ericads of absorbing water from a humid atmosphere. A gain always occurred on frosty nights and at times of high relative humidity. Both fresh and dried twigs of the evergreen ericads and of the conifers exhibited an ability to absorb water vapor from humid air. Such absorption is much greater in quantity per unit volume of leafy twig in the evergreen conifer Picea mariana than in the deciduous conifer Larix laricina, and in the scale-covered Chamaedaphne than in the other evergreen ericads. As such absorption not infrequently is three or four times as great as the transpiration during a very cold winter day, it is obvious how great is the advantage of this ability to lessen the demands upon the root and conducting systems at a time when the ground is frozen solid. 6. The transpiration per unit surface of the evergreen shrubs was very decidedly greater (4-10-30 and more times) than that of the deciduous shrubs during the winter under both outdoor conditions and indoor conditions which simulated warm spells in winter. 7. Among the evergreen ericads the relative rates of transpiration varied in the inverse order of their exposure and of their xerophytic structure. Chamaedaphne, which is the most exposed and the most xerophytic, has the lowest rates of transpiration and conduction. 8. During an average southern Michigan winter, protection by snow is not essential to the preservation of Chamaedaphne. In an extremely severe winter (1911-1912) absence of snow protection results in the killing of parts not so protected. 9. During the coldest weather (-29⚬) it did not appear that twigs or leaves of Chamaedaphne were frozen, as they were either perfectly pliable to handling or cracked as dry leaves. 10. Experimentation upon the rate of conduction by the lithium nitrate method showed a relatively higher rate at first, following the shock of cutting. Variations in external factors could not be arranged for a given plant because the shortness of the stem necessitated a short time of experimentation, and in order to obtain the data the stem had to be destroyed. 11. The rate of conduction was faster from warmer solutions, but was never zero when the solution was frozen. As the transpiration varied similarly, it seems obvious that the transpiration exercises a general regulatory function on the rate of conduction, similar to that which it exercises upon water absorption. The regulation is not exact, for in incipiently dry plants the rate of absorption and conduction is relatively greater than the rate of transpiration, while in turgid plants transpiration may be greater than absorption or conduction. 12. While the rate of conduction was relatively higher in the evergreen ericads than in other shrubs during the winter, leafless twigs of Larix exhibited virtually as high a rate of conduction, especially from colder solutions. 13. From the experimentation it appears that the rate of conduction is ample to the needs of these plants for temperatures above -15⚬ (and probably above -20⚬). 14. The transpiration of all plants experimented with in summer was very decidedly greater than in winter. This was most marked in deciduous species and least so in evergreen species. 15. Under summer conditions, hydrophytes exhibited the highest transpiration per unit area of leaf surface. In general, herbaceous plants transpired at a higher rate than shrubs. The more hydrophytic swamp shrubs transpired at a higher rate than the typical bog shrubs. The evergreen shrubs transpired at a very distinctly lower rate than the deciduous ones. Among the bog trees the lowest rates occurred in the evergreen species. The rate of the deciduous conifer Larix laricina was noticeably higher than that of the deciduous broad leaf tree Acer rubrum, and was decidedly higher than that of the evergreen conifer Picea mariana. 16. The rate of conduction under summer conditions was very high in comparison to that under winter conditions. This was less noticeable in the evergreen ericads than in other plants. The summer rate in Chamaedaphne was but 10-20 times that of the winter rate. The maximum rate obtained under conditions of experimentation which were not extreme was 23 cm. per hour in an evergreen ericad, while rates above 100 cm. per hour were frequently found in other shrubs and in herbaceous plants. An evergreen bog tree, Picea mariana, exhibited a distinctly lower rate of conduction than deciduous trees whether conifer or hardwood. 17. In the case of peat bog plants in nature, light (particularly sunlight) seems to be the effective factor in causing stomatal movements. Stomatal movements, while effective regulators of transpiration when they occur, do not appear so closely to regulate transpiration of peat bog plants as the evaporating power of the air. 18. In view of the fact that exposure to the very extreme summer conditions in 1911 and 1912 did not affect the vitality of the evergreen ericads, that neither did the average winter of 1910-1911, with its scanty snow covering during the coldest weather, while the extreme winter of 1911-1912 killed the parts of the evergreen ericads which projected above the snow; and in view of the fact that the evergreen habit is hereditary, that the position of the leaves in winter is different from that in summer, and that the transpiration is decidedly less than that of deciduous shrubs and of herbaceous plants in summer but greater in winter, the xeromorphy of these plants is real xerophyty, occasioned fundamentally by the necessity of protection when exposed to winter conditions and used advantageously by these plants during the summer.