30 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



Growth in Height. 



Over a dozen sahuaros were measured from time to time during a period 

 of from 2 to 4 years for the purpose of ascertaining the essential facts of 

 their longitudinal growth. A sharp-pointed style, made to slide along an 

 upright standard, on which was a metric scale, indicated the distance from 

 a fixed point at the base to the topmost point of the converging ribs, so 

 that the vertical growth during a given period was determined by sub- 

 tracting the measurement at the beginning from that made at the close of 

 the period; besides this, points were located at equal distances along some 

 of the furrows, and these distances were carefully measured from time to 

 time. Except at the apex, no lengthening of the stem was ever detected, 

 and the necessary conclusion follows that the longitudinal growth of the 

 sahuaro is strictly apical. 



Table 8. — Growth in height of Carnegiea gigantea. 



The longitudinal growth of nine individuals, measured as indicated, is 

 given in table 8. The period of measurement extended over nearly 5 

 years, but only Nos. 4 and 9 were measured for the full length of this 

 time. It will be seen that in most of these cases the growth is slow and also 

 irregular. The average growth for sahuaros over a meter in height is 

 about 12 cm. per year, but the very small ones grow very slowly; for ex- 

 ample. No. 24 grew only 4 cm. in 2 years. 



If we assume 10 cm. as the average rate of yearly growth, it follows that 

 the sahuaros on the laboratory domain which have attained a height of 

 from 1 to 10 meters (a large proportion of them fall within these limits) 

 have been growing where they now stand for periods ranging from 10 to 



