MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 331 



set was made from joints which had been formed during the pre- 

 vious year. Their development as buds began in March and April 

 and was nearly complete by June ist. Some enlargement may 

 ensue later in the season, or in the following season, as has already 

 been described.^" 



Swelling — Opuntia Sp. 



Mature Joints. 



(See Fig. 8.) 



(Joints of 1915-) 



Water HCIN/ioo NaOIIN/ioo 



Percentage. Percentage. Percentage. 



May 19, 1916 50.0 43.3 70.0 



" 26, " 40.0 36.6 . 52.1 



June 3, " 72.2 35-3 72.6 



" 13, " 23.9 53.6 551 



" 10, " 51-7 35-7 57-6 



Nov. 25, " 65.0 62.0 541 



(Measured) 47-6 50.0 35.5 



Jan. 28-30, 1917 37-6 34-3 36.0 



Feb. 20-21, " 12.3 9.1 10.3 



23-24, " 14.7 19.9 I9-I 



Mar. 27-28, " ii.o 10.9 ii.o 



Swelling of Other Joints Three Years Old. 



Water. HCIX'ioo. NaOH N/ioo. 



Per Lent. Per Cent. Per Cent. 



May 23, 1916 544 40-4 58.5 



Dried disks of percentage of original diam. ... 41.3 31.6 42.4 



The swelling capacity of sections appears to increase with 

 development and rising temperatures to June at which high values 

 were shown by both young and mature joints. A decrease during 

 midsummer is followed by a maximum reached in November. 



The average swelling of young joints was 31.2 per cent, in water, 

 28.9 per cent, in acid and 29.5 per cent in alkali for the season. 



The variations in swelling capacity during the second year are 

 indefinite but an average of the available records (seven tests) 

 shows 50.5 per cent, in distilled water, 45.2 per cent, in hundredth 

 normal hydrochloric acid and 56.7 per cent, in hundredth normal 



10 MacDougal, " Mechanism and Conditions of Growth," Mem. N. Y. 

 Bot. Garden, 6: 5, 1916. 



