Imbibition and Growth of Opuntia. 



137 



The size or swelling of any colloidal mass, such as a growing organ, 

 or particularly a joint of Opuntia, will depend upon the integrated 

 effects of several agencies, including transpiration, absorption, the 

 hydration coefficient (as determined by the acid, salt, and protein 

 content of the protoplasm), and the temperature. 



Several series of measurements of the separate processes involved 

 have been made at the Desert Laboratory. The earliest results of 

 importance with reference to transpiration as affecting growth were 

 obtained by Mrs. E. B. Shreve, who found that the actual amount of 

 water which an excised section of one of the cylindropuntias, Opuntia 

 versicolor (fig. 26), might contain begins to decrease some time after 

 midday and continues to do so until about daybreak of the fol- 

 lowing morning, then increases during the forenoon. 1 If the plant 



B 



Apr.ZO 



Apr.ZZ Apr.Z3 Apr.t4 



Fio. 26. 



A, tracing of an auxo- 

 graphic record of the 

 variations in length of 

 a stem of Opuntia ver- 

 sicolor. Below is a line 

 showing the daily vari- 

 ations in temperature. 

 Thegraph is composed 

 of sections of straight 

 lines representing con- 

 ditions of illumina- 

 tion. Interruptions 

 of the sunlight by 

 cloudiness are shown 

 on April 21, 22, 23, and 

 29. B, auxographic 

 record of variations 

 in thickness of stem 

 of Opuntia versicolor, 

 with temperature and 

 illumination indicat- 

 ed. (After Mrs. E. B. 

 Shreve.) 



was under conditions of satisfied imbibition capacity, the plotted line 

 showing such capacity would also be that of growth. Such a condition 

 does not exist, however. Also, if the simple capacity for imbibition 

 determined volume, and this capacity were always satisfied, the plotted 

 line of water capacity might be identical with that of the daily varia- 

 tion in volume of the mature organ. Neither does this condition 

 exist (fig. 27). 



The record of variations in volume of a joint from the time of its 

 beginning to maturity and through the following season, and selected 

 portions of this graph, are reproduced in figures 28 and 29. 



The aspect of the daily variations in volume shows seasonal altera- 

 tions and depends to some extent upon the age of the mature joint, 

 but it is evident that the joint begins to increase in volume in the 



1 See Shreve, E. B. An analysis of the causes of variations in the transpiring power of cacti. 

 Phyaiol. Researches, 2: No. 13. August 1916. 



