296 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



the present paper were made by an improved form of the auxograph 

 described by the senior author in 191 6. The changes in the instru- 

 ment were for the purpose of securing greater delicacy and accu- 

 racy. Twelve of these instruments as described on page 330 of the 

 present paper were available. 



The joints of Opiintia occupy the better part of a month in 

 developing from a length of 15 mm. with a volume of a few cu. cm. 

 to a length of 200 mm. with a volume of perhaps 150-200 cu. cm. 

 The entire mass of this member remains in an embryonic or elon- 

 gating condition until nearly mature, the development of woody or 

 permanent tissue being very light during the first 20-25 days. It 

 may be conceived therefore as a thick plate of protoplasts in all 

 stages of development from the earliest when enlargement is a result 

 of imbibition alone, to a state approaching maturity where the 

 osmotic action of the electrolytes in the vacuoles maintains a turgid- 

 ity indicated by the factthat expressed juice shows a possible pressure 

 of 5 to 8 atmospheres. Temperatures were established or taken by 

 thermometers with thin bulbs thrust into similar members in close 

 proximity, and as has been mentioned elsewhere in this paper, the 

 temperatures cited are those of the plant instead of the air as is the 

 case in many of the papers dealing with growth (Fig. i). 



A feature prominently emphasized by our studies is the interde- 

 pendence of effects. The influence of any one environic agency is of 

 course affected by the intensity of action of other agencies influenc- 

 ing the plant. This is well illustrated by the behavior of O. discata 

 No. 14, with respect to temperature. A young joint in the form of 

 a flattened naked bud of this plant was followed from Feb. 28, 1916, 

 to maturity, about April 30, 1916, and then its further alterations 

 in volume until June 7, 1916, at which time disks were taken and the 

 swelling capacity of the tissues determined. Measurements of 

 growth for every moment of 62 days, of reversible alterations 38 

 days and of final hydration capacity are available together with body 

 and air-temperatures. 



The plant stood on a cement bench near the glass of the southern 

 end of a greenhouse and exposed to normal illumination as modified 

 by the glass. It was kept in bearing with a precision auxograph 

 in such manner as to reduce errors to a minimum. The following 



