40 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



changes which occur when water is absorbed or given up. Some measure- 

 ments made from November 3, 1906, to January 29, 1907, during- a period 

 of expansion showed absolutely no changes except in the one dimension- 

 thickness. In this dimension, however, the joint not only changes with the 

 water- content, but also from year to year, gaining in thickness especially 

 as it bears other joints. It has seemed possible that the second period of 

 expansion, which occurred at a time when the sahuaros were contracting, 

 might be due to growth preparatory to the formation of flower-buds and 

 new segments (fig. 16). 



The number of buds borne by an Opuntia is-sometimes very great, but 

 this varies with the species. One medium-sized plant of Opuntia bore 

 247 flowers in a good year. They were borne on 56 joints. Table 11 

 gives some data as to the relative number of flowers and joints. Since 

 these data were collected before the species to which the plants belonged 

 had been identified, the name of the species is not given. 



Table 11. — Relative number of buds and segments on Opuntia sp. 



