FORM-ALTERATIONS AND GROWTH OF CACTI. 37 



While the furrows do become much flattened at the base, this is not such a 

 prominent feature as in the sahuaro. They are often partially spiral near 

 the top and in the older plants. 



The rate of growth in height is much slower than in the sahuaro, as is 

 shown by table 9. The records were obtained in the same manner as the 

 similar ones of the sahuaro. 



Plant No. 11, which was described on page 31, after growing 5 cm. in 

 2 years, suddenly increased 8 cm. between March, 1908, and October, 

 1909. This is the only instance observed of such rapid growth. No. 2 

 increased only 10 cm. in 4 years; No. 3 nearly 5 cm. in 2 years; and Nos. 

 4 and 5, 6 cm. or more in 3 years. Judging from the plants measured, 

 the average growth is a little more than 2 cm. per year. At this rate it 

 would require over 40 years for a plant to attain the height of a meter. 



PRICKLY PEAR (OPUNTIA SP.). 



While work on the Opuntia was carried on during several seasons, the 

 measurements of any one plant extended only through a single season, and 

 consequently the results, like those of the bisnaga, are of a fragmentary 

 nature. Naturally, too, the problems presented by the Opuntia are of a very 

 different nature from those of the sahuaro. It can not, in the nature of the 

 case, store such qtiantities of water, and its mechanical system is devel- 

 oped in an entirely different manner; but it is apparent that the joints do 

 have the capacity to store some surplus water and that the segments vary 

 in thickness under varying external conditions. 



In order to determine the relation of these conditions to the changing 

 thickness of the Opuntia joints, four plants were selected in the fall of 1906 

 and the thickness of the joints measured at marked places by means of 

 micrometric calipers from November 3, 1906, to May 25, 1907. Owing 

 to difficulties in always placing the instrument at the same angle, and to 

 the nature of the instrument itself, these measurements were not as accu- 

 rate as those made by dividers upon the sahuaro, but in using the figures 

 obtained anything less than a hundredth of an inch was discarded, and in 

 making the curves of expansion and contraction it is evident from their 

 similarity that their general trend can be relied upon and certain facts 

 established. 



Prickly pear No. 1 (^Opuntia discata') was measured four times on the 

 same joint. As might have been expected, the four series of measure- 

 ments were very similar throughout. On April 10 two small buds were 

 seen on the segment under measurement, and on May 15 they were within 

 a few days of flowering. On May 25 they had been picked off. 



On No. 2 {Opuntia toumeyi) six terminal joints were measured, each in 

 one place. Three joints were on the north and three on the south side of 

 the plant, but no difference was evident between the two sets. Budding 



