302 EDITH BELLAMY SHREVE 



during progressive desiccation and a higher one after subse- 

 quent rain or irrigation. 



When the plants are in dry soil their tissues are shrunken and 

 have distinct ridges and furrows around the tubercles. After 

 rain or irrigation, as well as before any desiccation has occurred, 

 the joints are plump and have many signs of being more turgid 

 than when in the desiccated condition. A test of the water- 

 content of joints one year old gave 18 grams of water per gram of 

 dry weight for joints in the high position and 10 grams for the 

 low one. These values were obtained by heating severed joints 

 to constant weight at 100°C. Ten separate determinations 

 were made and the results averaged. The individual varia- 

 tions were sometimes as great as 20%. 



It is evident that the movements thus far considered are due 

 to changes in turgidity. Measurements were also made on 

 Carnegiea gigantea, and on Opuntia fusicaulis, which show that 

 they have similar movements. Frequently joints continue the 

 growth of their vascular tissue while they are in the low positions 

 and when the rains come they are too stiff to return to their former 

 erect positions. Thus the curious final forms of the cacti are 

 determined by the amount of drought the plants experience 

 during the last stages of the secondary thickening of the joints 

 and not by any peculiarity of the growing points. 



When soil conditions remain fairly constant no observable 

 change of position takes place, provided the measurements are 

 made at the same time each day. However, when measurements 

 are made at 2 to 12 hour intervals another phenomenon appears 

 which may be said to be superposed upon the nearly constant 

 position in soil of unchanging water-content, as well as upon 

 the general downward or upward motion in soil of decreasing 

 or increasing water-content. This new movement takes place 

 within each 24 hours and may be described briefly, as consist- 

 ing of a downward movement at night and an upward one 

 during the day. Certain exceptions to this general rule occur 

 and will be considered. The character and amount of this 

 motion, together with a general description of the accompanying 

 environmental conditions, will be seen from the following 

 experiments. 



