FORM-ALTERATIONS AND GROWTH OF CACTI. 27 



1906) and that of the greatest expansion (March 9, 1907).amounted to 27.3 

 cm. and at the lower to 8.1 cm., showing a difference of 19.2 cm. in the 

 expansion at these two points, the upper part expanding nearly three times 

 as much as the lower one. As stated, the plant in question was 6 meters 

 high, and as the variation increases instead of decreasing toward the top, 

 it will be well within the limits of safety if we assume the variation at 2 

 meters as the average. 



At this height, at the time of greatest expansion, it measured 1.713 

 meters in circumference. This gives a diameter of 54.532 cm. At its 

 minimum the circumference was 1.44 meters, giving a diameter of 45.8 

 cm.; that is, there is a difference of 8.732 cm. in the diameter at maxi- 

 mum and minimum. This represents the gain of a hollow cylinder of 

 water measuring 1.713 meters on its outer and 1.44 meters on its inner 

 circumference, with a thickness of 4.362 cm. and a height of approximately 

 6 meters, or a volume of 412.393 liters of water, which were absorbed and 

 stored between November 14, 1906, and March 9, 1907. In addition to 

 the amount lost by transpiration, which would not be inconsiderable, this 

 calculation does not take the branches into consideration, so that the total 

 amount taken up must have been much more than the amount given. 



A general review of the observations shows that the water-balance is 

 depletedveryslowly by transpiration, and that the loss is quickly regained 

 in the rainy season. Thus- the circumference of sahuaro No. 11 de- 

 creased at the rate of 4.3 mm. per day from March 6 to June 1, the dry 

 foresummer, but increased at the rate of 6 mm. per day from November 17 

 to December 15, 1906. After the last date the rate fell to zero in March, 

 1907, when a shrinkage set in, which reached a rate of 2.3 mm. per day 

 on May 25, 1907. 



The rate of water-loss from the flowers by transpiration, however, has 

 been found to be very rapid. The flowers are produced in large numbers 

 immediately below the apex of the trunk, where they form a conspicuous 

 crown, including several hundred in favorable seasons. Relatively few are 

 open at any one time. The production of buds and opening of flowers 

 begin on the southeast side, and from there advance to the northwest, 

 until the top is nearly or quite encircled, the process requiring several weeks 

 for its completion. It has been observed that the number of flowers is 

 much greater in wet than in dry years, and that they are especially numer- 

 ous on individuals of the giant cactus growing where there is a constant 

 and abundant water-supply. 



It is evident that during the period of flowering an additional demand is 

 made upon the water that the plant has stored during the winter rainy 

 season. In order to form some estimate as to the extent of this, some de- 

 terminations were made of the actual transpiration of the sahuaro flowers 

 and buds in the summer of 1907. These were cut from the plant and the 

 cut end at once covered with vaseline to prevent evaporation; they were 



