MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 295 



of great volume which is active through a long period. Such plants 

 are amenable to chemical analyses, and have mechanical qualities 

 which make it possible to place the apex in bearing upon an auxo- 

 graph lever and secure a continuous record of its activity during 

 the entire period of enlargement, as well as of the subsequent varia- 

 tions in length. Detailed studies of the course of transpiration and 

 respiration of these plants have been made at the Desert Labora- 

 tory, and the available information on these subjects was of great 

 usefulness in interpreting growth and other changes in volume. A 

 cylindropuntia was also tested in order to ascertain possible differ- 

 ences due to mechanical form. Both kinds have a type of respiration 

 in which a notable accumulation of acids occur at temperatures in 

 the lower part of the tonic range and in darkness. The leaves of 

 Mesemhryanthemum presented different morphological features, 

 but a similar type of respiration. The massive globose and cylin- 

 drical stems of Echinocactiis and Carnegiea were also used as their 

 metabolism is of a character which does not result in any notable 

 accumulation of residual acids in any part of the respiratory mesh. 

 The meristem region in both is entirely terminal, and some detailed 

 studies of the fate of the carbohydrates and of the non-auxetic 

 variations in thickness and length as well as of transpiration had 

 been previously made. 



Growth of Opuntia. 



These preliminary studies brought out the fact that the flattened 

 joints of the opuntias undergo most rapid growth during the day- 

 light period, coincident with decreasing acidity and lessened trans- 

 piration, and that actual shrinkage occurs in maturing joints as the 

 result of reactions which are masked during the period of most 

 active growth. The entire development of about forty flattened 

 joints has been followed from bud to maturity, and the changes in 

 volume of members in an adult condition have been noted for long 

 periods under varying conditions. The swelling of hundreds of 

 specimens from growing and mature joints were measured, and an 

 extended series of records of the action of gelatine, agar, albumen 

 and cactus mucilage in acids, alkalies and salt solutions made. 



Unless otherwise stated, all of the growth records included in 



