28 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



then weighed, and weighed again at the end of an hour, during which they 

 were left in the open air in full sunlight. Table 7 shows the amount of 

 water lost. 



Table 7. — Loss of water by transpiration in flowers of sahuaro. 



The average transpiration of a fully opened flower, as shown by these 

 weights, is 851 mg. for an hour of full sunlight. Each flower lasts but one 

 day, but a single plant may bear hundreds, and the transpiration of the bud 

 must go on for many days; so it is clear that the water lost by the plant 

 during the flowering- season is no small amount, and, moreover, in esti- 

 mating the amount of water used by the inflorescence we must add that 

 used in the tissues of the flowers themselves. 



I'he dry weight of one flower was found to be 13 .2 per cent of its weight, 

 leaving 86.8 per cent as the water- content. Estimating from the average 

 of the flowers weighed, about 3.27 liters of water are stored in the tissues 

 of every 100 flowers. 



Notwithstanding these amounts, which in themselves seem very consid- 

 erable, they form such a small portion of the entire amount stored in the 

 plant that no difference has ever been detected by the measurements in the 

 contraction of flowering and non-flowering plants, though the flowers are 

 mostly formed after the end of the winter rains, when there is compara- 

 tively little water in the soil. It is worth noting, however, that the plant 

 does not flower until it has attained considerable size, and that, as has 

 already been pointed out, the abundance of flowers is directly correlated 

 with the amount of available water in the soil. 



iJROWTH. 



But little evidence has been obtained as to the relation between the 

 growth of the sahuaro and rainfall. Growth in length occurs during the 

 time of the summer rains, and these have not failed for a single season 

 since the observations were begun. There is, however, a single instance 

 where a plant under observation was deprived of apart of its water-supply. 

 Sahuaro No. 9 (plate 5) grew close to the laboratory and at times received 

 some water from artificial sources. Between April, 1905, and January, 

 1906, it increased 19 cm. in height. This was the greatest growth which 

 any measured plant attained during that summer. The following summer 



