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* Numbers 102 to 108 are out of their regular places. 102 should go after 9, 103 should go after 21, 104-106 should go after 40, 107 should go after 72, and 108 should go after 90.
This includes many genera widely distributed in the region east of the Cordillera of Bogota which west of them are found only in the Magdalena.
GROWTH IN TREES.1
By D. T. MACDOUGAL.
(Read April 21, 1921.)
My studies of the course and physical conditions of growth were extended to include the changes in circumference and diameter of tree trunks as well as the elongation of growing branches in 1918, and a new technique with specially designed instruments was found necessary for the analytical study of the changes in volume of these massive organs. The records are now continuous for a large number of trees for many months, one tree having been under continuous measurement since September, 1919.2
Two new instruments were designed for obtaining measurements of growing trees. The dendrograph, an instrument for making continuous records of the variation of tree trunks, is an instrument consisting of a floating frame of metal of low temperature coefficient, such as invar or bario, which may be placed around a tree trunk, and the variation in distance between a contact rod on one side of the trunk and of one end of a rod or lever on the other side is traced by a pen on the free end of a lever on a sheet of paper carried by a recording cylinder. Such measurements are in terms of the diameter. In an earlier form of the instrument two levers were employed. One end of a bearing lever was placed in contact with the tree, the free end being linked to the short arm of the recording lever. The results previous to October, 1920, were obtained by
1 The full paper of which this is a synopsis will be published by the Carnegie Institution of Washington as prospective No. of its series.
2 MacDougal, D. T., "The Dendrograph; a New Instrument for Recording Growth and Other Variations in the Dimensions of Trees," Carnegie Inst. Wash. Year Book for 1918, pp. 59-60. "The Dendograph," ibid., for 1919, pp. 72-78. "The Coure of Growth in Trees as Measured by the Dendrograph," ibid., for 1920, pp. 51-52. “Measurement of a Season's Growth of Trees by the Newly Designed Dendrometer," ibid., for 1920, p. 52.
this type of instrument. The older and the improved lever sets are illustrated in Plate I, Figs. 1 and 2.
A dendrometer of simple design has been perfected which may be placed around the trunk of a tree and the size of the trunk read on a dial from time to time. The essential parts of this instrument are an encircling wire engaged with a number of bearing levers. One end of the wire is anchored and the other is attached to the short end of a lever, the free end of which moves over a scale giving readings of the size of the trunk in terms of several radii, or of the circumference.
By the assistance of collaborators measurements of a number of evergreen and deciduous trees in various habitats from the Atlantic seaboard to the Pacific coast were made in 1919 and 1920. Beech, ash, walnut, sycamore, pines, spruce, fir, poplar and oak trees were included in the list.
The principal generalizations supported by the information obtained may be briefly summarized as follows:
The period in which enlargement of trunks takes place is comparatively brief even in places in which the season is of indeterminate duration. Growth is an activity of an embryogenic tract of tissue, the activity of which depends upon environmental conditions, and no part of the observations suggested a seasonal rhythmic action. The Chihuahua pine which exhibits growth of the trunk with that of the branches on the dry mountain slopes with the advance of the temperatures in May and June, is brought to rest coincident with the desiccation of the soil in the dry fore-summer. Reawakening ensues consequent upon the summer rains and enlargement continues until checked by the decreasing temperatures and increased soil desiccation in the autumn.
The Monterey pine (Pinus radiata) shows beginning growth of the trunks with the advance of temperatures January to April, and comes to rest in July with the desiccation of the soil. Quercus agrifolia in the same region begins earlier and ceases to grow in June or July. Both may be awakened in July or August by deep irrigation of the soil (Fig. 3).
The trunks of all the trees measured show a daily variation in size, by which the maximum is reached shortly after sunrise and
the minimum at a time after noon, dependent upon external agencies. These variations appear to depend upon the water-balance in the woody cylinder, are greatest in the seasons in which water-loss
FIG. 3. A, dendrographic record of variations in diameter of Monterey pine tree I meter from the ground for week beginning April 5, 1920. B, record from instrument attached to the trunk 9 meters from the ground. Daily equalizing variations with actual enlargement beginning mid-week X 8, on a scale of 10 mm. intervals.
from the crown is greatest, are least in the cooler or damper seasons, and are to be detected in the records even in the period of most rapid enlargement of the trunk (Fig. 4).
6PM 6AM 6PM 6AM 6 PM 6AM 6PM 6AM 6PM 6AM 6PM 6AM 6PM 6AM
FIG. 4. Auxographic record of elongation of terminal internode of young pine tree showing cessation of growth and shrinkage during the midday period, coincident with the decrease in diameter of trunks of larger trees, X3, on a scale of 10 mm. intervals.
The trunk of a tree may in fact be compared to the supply hose of a fire engine coupled to a hydrant. When the pressure from the mains is enough to supply water faster than it can be pumped out the hose is distended. When the engine tends to take water faster than it would be delivered by the system, the hose would tend to