8 MACDOUGAL— GROWTH IN TREES. 



this type of instrument. The older and the improved lever sets are 

 illustrated in Plate I, Figs, i 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 ob- 

 tained may be briefly summarized as follows : 



The period in which enlargement of trunks takes place is com- 

 paratively brief even in places in which the season is of indeter- 

 minate 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 ad- 

 vance of the temperatures in May and June, is brought to rest coin- 

 cident with the desiccation of the soil in the dry fore-summer. Re- 

 awakening ensues consequent upon the summer rains and enlarge- 

 ment continues until checked by the decreasing temperatures and 

 increased soil desiccation in the autumn. 



The Monterey pine (Pinns 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 agri- 

 folia 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 



