44 CARNEGIE INSTITUTION OF WASHINGTON. 



GROWTH AND HYDRATION. 



Growth in Trees, by D. T. MacDougal. 



The changes in volume of trunks of diverse types of trees, deciduous 

 and evergreen coniferous types native to the Atlantic seaboard, the 

 Mississippi Valley, various elevations in the Rocky Mountains, the 

 plateau, mountain slopes, and desert valleys of Arizona, the plains of 

 southern California, and the coastal region at Carmel, have been 

 measured for several years as a part of a comprehensive research upon 

 the fundamental physical factors in growth. An improved type of 

 the newly designed dendrograph described in Publication No. 307 has 

 been used and the following generalizations upon the basis of informa- 

 tion acquired have been established : 



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

 paratively brief, even in places in which the season is of indeterminate 

 duration. 



2. Growth is an activity of an embryogenic tract of tissue, which 

 depends upon environmental conditions, and no part of the observa- 

 tions suggested a seasonal rhythmic action. The Chihuahua pine, 

 which exhibits growth of the trunk with that of the branches on the 

 dry mountain slopes in 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 de- 

 creasing temperatures and increased soil desiccation in the autumn. 

 The Monterey pine (Pinus radiata) shows beginning growth of the 

 trunks with the advance of temperatures, Januarj^ 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. 



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 from 

 the crown is greatest, are least in the seasons of lower temperatures 

 and high humidities, and are to be detected in the records even in 

 the period of most rapid enlargement of the trunk. 



4. Measurement of variations in the woody cylinder were taken by 

 arranging the contact rods of the dendrograph to bear on the wood 

 formed by the tree two years previously. Thus in 1920, holes were 

 bored through the wood of that year and of 1919 and contacts made 

 at the bottom of the cavities. 



5. In general, the awakening and growth of the teiTninal buds, 

 with resultant elongation of leaders and branches, begin in many 



