FORM -ALTERATIONS AND GROWTH OF CACTI. 



By E. S. Spalding. 



SAHUARO (CARNEGIEA GIGANTEA). 

 CONDITIONS AND METHODS OF MEASUREMENT. 



In a former article* it was shown that the giant cactus not only pos- 

 sesses a structure remarkably fitted for the storage of a large quantity of 

 water, but also, without the slightest interference with the efficiency of its 

 mechanical system, promptly adjusts itself by a change of form to the 

 increased supply taken up from the soil after a rain, and to its diminution 

 during subsequent periods of drought. The interest attaching to this 

 plant, presenting as it does the most conspicuous and perhaps the most 

 perfect arrangement for water-storage yet developed in any of the plants 

 of the region to which it belongs (plate l), together with its fine me- 

 chanical adjustment, the peculiarities of its root-system, and the correlation 

 of its structure and habits with what has been observed of its local and 

 general distribution, appeared to justify the continuance of the work pre- 

 viously undertaken, with the hope that a longer series of measurements 

 might contribute something more definite regarding certain questions that 

 were not satisfactorily settled. Accordingly the study has been continued 

 at intervals, the whole period covered being nearly five years, in the 

 course of which the influence of certain external and modifying factors has 

 been more exactly determined, rate and mode of growth have been ascer- 

 tained, and comparative studies of some other species of cacti have been 

 made. 



As regards the sahuaro, the methods already described have been em- 

 ployed without essential change. Points were located in pairs on opposite 

 sides of a furrow or rib and marked with India-ink. Distances between 

 the points were measured at stated intervals with a pair of dividers pro- 

 vided with a micrometer screw, so that slight movements might be accu- 

 rately noted (plate 2). In this way changes in width of furrows or thick- 

 ness of ribs on different sides of the trunk and at different heights were 

 observed and correlated with records of rainfall at the Desert Laboratory, 

 all the plants under observation being located within a few rods of the 



*Spalding, E. S. Mechanical Adjustment of the Sahuaro (Cereus giganteus) to Varying 

 Quantities of Stored Water. Bull. Tor. Bot. Club, 32, 57-68, 1905. 



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