ELONGATION 01" THREE TRUNKS, ETC. Ill 



the trunk itself, and if at any time it is cut down 

 and examined while yet sound, the remains of the 

 first-formed limbs may still be found near the 

 center in the form of knots at the same height at 

 which they were originally produced. One who 

 doubts can easily satisfy himself that the trunk 

 itself does not elongate by the following experi- 

 ment: Drive a number of nails or tacks in the 

 smooth bark of a tree, at equal distances apart, one 

 above the other. Measure their distance apart 

 when first driven, and at intervals of a few months, 

 and after the lapse of several years. To be very 

 accurate file a line across the head of the nail or tack, 

 so that there will be no mistake in measurement. 

 Probably no difference will be found in the measure- 

 ments from time to time. Professor Asa Gray, 

 who suggested this test, found the measurements 

 the same in April and at the close of the growing 

 season in August. The trees he measured were 

 young saplings of Magnolia, Buckeye, and Yellow- 

 wood. A similar series of observations was made 

 at the Iowa Agricultural College with the same re- 

 sults. In this connection the question will prob- 

 ably be asked in what parts of plants does growth 

 in length take place? No brief answer to this 

 question can be given that will cover all cases. In 

 the chapter on sap circulation it was noticed that 

 growth in diameter in our ordinary trees takes place 



