Plant-geography upon a physiological basis, Part 1 69-70 (1903)
Andreas Franz Wilhelm Schimper,
(trans) William Rogers Fisher, (expanded and edited) Percy Groom, Isaac Bayley Balfour
I. ATMOSPHERIC PRESSURE.
As Wieler and Jaccard have shown, the pressure within the stratum of the atmosphere in which plants grow does not at all correspond to the absolute optimum pressure for the growth of plants. On the contrary, a diminution of the partial pressure of the oxygen—for the latter only, and not the total atmospheric pressure, comes into question—occasions an acceleration in growth until a certain low pressure is attained, which is constant for each species, and beyond which any further diminution in pressure causes a retardation in the rate of growth. We find this absolute optimum atmospheric pressure for growth to be in the case of Helianthus annuus about 100 mm., but for Vicia Faba about 200 mm. Again, an increase in atmospheric pressure above 760 mm. (or the corresponding pressure of oxygen) up to about 2 1/2 atmospheres occasions a retardation, but after that an acceleration in growth. There are therefore for growth two absolute optima of atmospheric pressure, both of which differ considerably from the pressures that prevail in the inhabited stratum of air, the one being at a far lower, and the other at a far higher oxygen-pressure.
According to Jaccard a decrease in the pressure of oxygen occasions not only more rapid growth, but also richer branching in the axes and roots, as well as an increase in the size of the leaves. Jaccard's following tabular statement shows how considerably growth is favoured by rarefaction of the air:—
UNDER DIFFERENT CONDITIONS OF ATMOSPHERIC PRESSURE (after Jaccard).
(R. represents growth in air at a pressure of 15 cm.; O. at the normal atmospheric pressure.)
|1. Jerusalem artichoke. Tubers with shoots 1 cm. long, in 8 days||40 cm.||4-5 cm.|
|2. Vicia Faba, 3-4 cm. high, in 8 days||22 “||0.8 “|
|3. Oxalis crenata, tubers with two tall shoots||35 “||3.5 “|
|4. Bellis perennis, plant 3-4 cm. high, in 15 days||10 “||6 “|
|5. Violet, plant 3 cm. high, in 15 days||8 “||6 “|
|6. Onions, with shoots 3-3 1/2 cm. high, in 10 days||16 “||6 “|
When air less rarefied than the above is used, correspondingly less marked results follow, but in spite of the greater importance of moderate atmospheric rarefaction in relation to plant-life, this has secured scarcely any attention from Wieler and Jaccard. A pressure of 35 cm. was employed in one solitary experiment conducted by the latter observer on wheat-seedlings. The plants experimented on attained in twenty-three days a length of 20 cm., instead of 17 1/2 cm. at ordinary atmospheric pressure.
Air that is as rarefied as that used in most of Wieler's and Jaccard's experiments occurs in nature only at the highest summits of the Himalayas, for example at an altitude of 8,839 meters on Gaurisankar, where, if we assume that the temperature of the air at the sea-level is 25°, a pressure of 26 cm. prevails. An atmospheric pressure of 35 cm., under which the above-mentioned experiment with the wheat-seedlings was carried out, corresponds to an altitude of about 6,000 meters, at which, in Thibet, a stately flowering plant, Saussurea tridactyla, has its normal habitat: this plant is discussed and illustrated in the section of this book dealing with alpine vegetation. There appears therefore to be no reason why some vegetation, even if only cryptogamous, should not occur at still greater altitudes. In any case however, judging from the information available, there are very few plants occurring at altitudes such that the rarefaction of the air would cause their growth to be appreciably more rapid than in the low land. For a definite reply to the question we must await the results of experiments on typical alpine plants.
The variations in the atmospheric pressure at different altitudes, even if not directly, yet indirectly are of vast physiological importance, since humidity, temperature, and light depend upon their magnitude. The changes that these last-named factors undergo owing to a reduction in atmospheric pressure are responsible for the influences exerted by the alpine climate on vegetation, as will be described further on.