100 CARNEGIE INSTITUTION OF WASHINGTON. 



and the plant in turn resists the pull from the air, cutting down the 

 transpiration rate, while at the same time it has an increased ability 

 to take up water to replace the amount lost. The increased water- 

 content of the tissues coincident with minimum acidity is in harmony 

 with the results of Mr. E. R. Long, who found that the hydratation 

 of tissues of cacti was less in dilute acids than in distilled water (see 

 p. 92). This result is also in agreement with the measurements of 

 growth by Dr. MacDougal (not yet published), which show maximum 

 rate of enlargement at the time of least acidity. It does not conform, 

 however, to the conclusion of Fischer, Borowikov, and others on the 

 influence of low acidities on hydratation. 



The measurements of water-content and absorbing capacity show 

 a slight tendency toward a movement of the water within the stem 

 from the center toward the periphery during the day, and a return 

 toward the center at night, thus making an outer supporting cylinder 

 of turgid cells during the day, which would tend to the erection of 

 stems, but would probably not be great enough to be the sole cause 

 of the movement. It is also probable, from experimental evidence, 

 that the rise is further aided by the increase in turgidity caused by the 

 absorption of heat energy from direct sunlight. But evidence from 

 exceptions to the usual behavior points to the changes in water- 

 content as being the most important cause of the motion. 



VARIOUS SPECIAL INVESTIGATIONS. 

 Fruit Development in the Cactacece, hy Duncan S. Johnson. 



The continuation of experimental work on Opuntia fulgida shows 

 that detached fruits, when placed on damp soil in the green-house or 

 laboratory, give rise to roots 5 cm. long in 5 weeks' time. No shoots 

 were evident at this time on these fruits. Shoots planted in a green- 

 house had shoots 3 cm. long after a year's growth. While flowers 

 arise, on the attached fruits, only from areolae near the top, vegetative 

 shoots may arise, on detached fruits, from areolae at the base, middle, 

 or top, depending on the position of the fruits in the soil. Roots may 

 arise from any areola placed downward in the soil or may even develop 

 independently of an areola on the margin of the perianth scar at the 

 top of the fruit or close to the scar of the broken off fruit-stalk at its 

 base. That is, a root may evidently arise at any point on the fruit 

 where it can push through the epidermis. Data are being gathered 

 to aid in determining the cause of the persistent adherence of the 

 fruits of this choUa and of the cause of its final dislodgment. 



A study of the development of the fruit, seed, and embryo sac of 

 0. leptocaulis is being made for the sake of comparison with the cholla 

 and as a basis for experimental work on this species at Tucson. In 

 most of the features mentioned, 0. leptocaulis shows rather close simi- 



