6 GROWTH IN TREES. 



Monterey pine (Pinus radiatd) 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. 



7. 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 cooler or damper seasons, and 

 are to be detected in the records even in the period of most rapid 

 enlargement of the trunk. 



8. 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. 



9. In general the awakening and growth of the terminal buds with 

 resultant elongation of leaders and branches begins in many trees some 

 time before enlargement of the trunk takes place. The period sepa- 

 rating the two may be no more than a week in Quercus agrifolia 

 and has been seen to be as much as 10 or 12 weeks in Pinus radiata. 

 Observations on the Parry spruce and Douglas fir show that the trunks 

 of these trees are enlarging at a time when the buds are in a very 

 early stage of enlargement. 



10. In the single case in which dendrographs were attached to a 

 pine tree 1 meter and 8 meters above the ground, growth began coin- 

 cidentally at the two places in 1920. In the following year, however, 

 the dendrograph at the higher point on the trunk recorded enlargement 

 a few days before any action near the ground was made visible. In 

 February 1921 an auxograph was brought into bearing on the inter- 

 node of a pine tree 5 or 6 years old which had been formed in 1919. 

 The buds had made a growth of 4 or 5 cm., but no action had yet 

 begun in the internode. A second instrument was brought into 

 bearing on the middle of the internode formed in 1920 on another young 

 tree. Steady enlargement was in progress. 



11. The embryonic layer of a tree is in the form of an inclosing sheath 

 terminating in the cones of the growing points. Activation of this 

 tract is generally initiated in the growing points. Swelling in the 

 cambium layer may be practically coincident with this awakening in 

 some trees. Cases are recorded in the present paper in which weeks 

 elapsed between the awakening of the buds and the enlargement 

 of the base of the trunk. Activation of the growing cells may be taken 

 to depend upon the localized food-supply, temperature, moisture, or 

 other factors. 



