FORM-ALTERATIONS AND GROWTH OF CACTI. 



41 



The proportion of the number of flowers and young joints to the entire 

 number of segments in the plant indicates that there must be a considera- 

 ble drain upon the water stored in their tissues to support transpiration 

 and to form the new tissues. To gain a more exact knowledge of the 

 amount of water required for these purposes, weighings of flowers and buds 

 were made, as in the case of the sahuaro. Table 12 shows the amounts 

 transpired in an hour by the different parts of the Opuntia, together with 

 the dry weight and percentage of water in two flowers and joints. 



Table 12. — Loss of 



by transpiration, and water-content of flowers, buds, and segments 

 of Opuntia. 



Original 

 weight. 



Loss. 



Time. 



Dry 

 weight. 



Percent- 1 



age of 

 water in 



original 



weight. I 



Flower 



Do 



Do 



Bud 



*Joint of present season 

 Joint I year old at 

 least 



ems. 



23-570 

 23-677 

 23-949 

 17-542 



III.6SS 

 118.790 



260 

 262 

 279 

 187 

 20s 



233 



1 2" IS' 



1 45 

 3 10 



2 45 



' to i^is" p. m. 



2 45 P- m- 

 4 OS P- ni- 



3 45 P- m- 



ems. 

 3-281 

 3.222 



I 20 2 20 p. m. 22.690 



3 00 p. m. 37-095 



86 

 86 



82 



♦Approximate surface 306.15 sq. cm. 



It will be seen from table 12 that the proportion of water in the flowers, 

 buds, and plant itself are not far from that in the sahuaro. The average 

 transpiration of a flower in an hour of full sunlight, as shown by these 

 weighings, is 267 mg., and that of a joint seems to be nearly as much. 

 The transpiration of No. 10, table 11, would be 67 c.c. per hour of full sun- 

 light if all the flowers were open at once. But according to the percent- 

 age of water in the plant itself, one of the joints would contain about 100 

 c.c. of water, and the plant in question only had 119 joints previous to the 

 flowering season. It is likely that this estimate of the water in the joints 

 is low, for it was made in May, after they had been contracting for some 

 time, but at the best it is evident that the roots of the Opuntia must, in 

 such cases as the foregoing, be able to draw some water from the soil, 

 even in a dry time, to make good the loss from transpiration. 



