FORM-ALTERATIONS AND GROWTH OF CACTI. 39 



is a striking phenomenon, familiar to even a casual observer. But a care- 

 ful study of the measurements revealed some other facts and some points 

 of difference between the response of the Opiintia joints and that of the 

 sahuaro trunk. 



When the measurements were first made it was at the end of a long; 

 dry period, no rain sufficient to wet the ground having fallen for over 

 two months. A few drops did fall on October 29, five days before the 

 first measurement, but it lasted only a few minutes and cleared, leaving 

 the ground dry. But in the following two weeks, before any rain came, 

 the segments, already much shrunken, showed in many cases a gain in 

 thickness (fig. 16). 



After the rains came the plants responded by a considerable increase in 

 thickness, but, like the sahuaros, they seemed to reach a limit of expansion 

 soon after the ground was thoroughly soaked, and then not to increase to 

 any marked degree. But the Opiintias (with the possible exception of 

 No. 4) showed a second period of expansion which began early in Febru- 

 ary and continued a month or more, although there had been very little 

 rain. The joints reached their fullest expansion when the buds of the 

 flowers and new joints began to develop- The beginning of the dry pe- 

 riod in the season during which the measurements were made was almost 

 coincident with the beginning of the contraction of the Opuntia segments, 

 and this corresponded very closely with the time of contraction of the 

 sahuaro trunk . The fact that at this time buds were developing most rapidly 

 might of course be a factor in the ensuing shrinkage, but there was no 

 evidence to support this inference, for the measurements showed no dif- 

 ference between the segments that bore buds and those that did not. 



There was no apparent difference between the expansion of the joints 

 on different sides of a plant, but in case of a series of joints the older 

 (lower) ones expanded more than the younger ones. The three lower 

 segments became too thick for measurement with the instrument, the 

 limits of which were 1 inch, so that the exact amount of expansion is uncer- 

 tain; but ignoring all beyond an inch (which is considerable), the lower 

 three joints expanded 30 units more than the upper three. (In speaking 

 of the Opuntias a unit represents 0.01 inch.) 



The manner of growth of the Opuntia segment is told in plate 6, which 

 shows the joints in different stages of development . Their period of growth 

 is in March and April. The new joints are, for the most part, borne on 

 the terminal ones of the plant, but they may also be produced on the older 

 joints. Table 10 shows the growth in length and width of four segments 

 from March 27 to April 10. It is evident that the growth is soon com- 

 pleted, as segments 1 to 3 grew 3.35, 3.97, and 4.92 cm. in length in 13 

 days, while No. 4 increased 6.8 cm. in 19 days. 



When the young joint has become fully expanded its growth in length 

 and breadth practically ceases and these dimensions do not share in the 



