Imbibition and Growth of Opuntia. 143 



difference between the amount of swelling of sections in distilled water 

 taken at sunrise and swelled at 20 and 28 C. was so small as to be 

 negligible. The same condition prevailed in dried slices taken in the 

 morning and dried rapidly in the desiccator. Slices which came down 

 to a thickness of about 0.6 mm. swelled 392 per cent at 20 C. and a 

 second set increased an equivalent proportion at 27 to 28 C. The 

 sets of living sections used for these tests gave an increase of about 6.5 

 per cent in water. Sections taken at noon swelled 8.2 per cent at 20 C. 

 and 9.8 per cent at 28 C. Sections taken at the end of the day 

 swelled 4.6 per cent at 20 C. and 6.5 per cent at 28 C. The material 

 used for these paired tests was selected to be equivalent as far as pos- 

 sible, and comparisons transverse to those stated are not allowable. 



The acidosis produced by the residual acids of the morning condition 

 may be taken to be accentuated by the rising temperature and to 

 cancel or mask any additional absorption which might accrue as a 

 direct effect of temperature on a neutral solution. The increases at 

 other tunes would be due to the direct action of rising temperatures 

 on absorption. 



It became evident in the earliest work done with the opuntias that 

 the young joints showed a swelling in hydroxid solutions greater than 

 in water or acids, a fact probably attributable to the formation of 

 compounds of the sodium hydroxid with the carbohydrates present, 

 in conjunction with possible sodium albuminates. The carbohydrates 

 are in greatest proportion in young joints. The swelling in hydroxid 

 in later stages comes down to a level near that in acid and in water. 



An additional fact of interest was the action of the juice of the joints 

 taken in the condition in which it is found at midday. Duplicates of 

 the dried slices, which have already been noted as showing an increase 

 of 390 per cent in distilled water, swelled 325 per cent at 20 C. in the 

 expressed juice. Similar dried slices swelled 372 per cent in the freshly 

 expressed juice of Echinocactus wislizeni. Its hydrating effect on thin 

 plates of a biocolloid consisting of agar 6 parts, opuntia mucilage 2 

 parts, bean protein 1 part, and gelatine 1 part was much marked. 

 Such sections swelled 2,450 per cent at 20 C. in distilled water, and 

 only 700 per cent in the fresh juice of Opuntia taken at midday. 



The principal factors which influence the rate and course of growth 

 of Opuntia have been described in the preceding pages. The joints 

 when in the youngest stage have a mode of growth which does not 

 differ materially in the record which it makes from that of many other 

 green plants. As soon as it reaches medium size its innate peculiarities 

 of transpiration and metabolism and certain morphological features 

 operate to give it a highly characteristic daily chart of elongation and 

 retardation or shrinkage. The respiration of these plants is of a 

 character which results in the accumulation of acids to an amount 

 equivalent to as much as 0.1 N malic acid at daybreak, which is suffi- 



