58 Spalding : Mechanical adjustment of suaharo 



ring of fibro-vascular bundles is made up of thin-walled water- 

 storing cells. 



From the construction it seems evident that a change in bulk 

 corresponding to varying quantities of water contained in this tissue 

 could hardly affect the central mechanical cylinder; but would be 

 far more likely to manifest itself externally by expansion or con- 

 traction of the circumference, effected by folding or unfolding of 



the ribs and furrows. 



To determine whether this actually takes place, measurements 

 were made by marking with India ink points opposite each other 



meas 



tance between them at intervals of a day or more. Opposite 

 points at the bases of the ribs were also marked in the same way, 

 and the thickness of the ribs between these points measured by 

 callipers. Finally a wire was placed around some of the plants, 

 with a coiled spring, partially stretched, inserted between its ends. 

 Variation in the length of the spring thus indicated any change 

 that took place in the circumference. It was found that these three 

 sets of measurements always corresponded. The ribs and furrows 

 expanded and contracted at the same time, and their action was 

 coincident with a corresponding increase or decrease in the circum- 



ference. For the sake of convenience, the comparisons which 



follow will be restricted to the measurements of the furrows. 



To exhibit their changes graphically, curves were constructed, 

 distances between ribs being laid off on the vertical axis, each 

 space representing J^ inch, and time being represented on the 

 horizontal axis, each space representing one day. (In the ac- 

 companying diagrams every fifth line only is shown, so that each 

 vertical space represents ^j inch, and each horizontal space 5 

 days.) The English instead of the metric scale was employed, 

 merely because the finest rule available at the time was in the 

 former system. The measurements were all made with blunt cal- 

 lipers, and often when the cacti were swaying in the wind, so that 

 absolute accuracy to J^ of an Inch was not always attainable; for 

 all the comparisons made, however, it is believed that the curves 

 are substantially correct. 



The changes In width which the furrows exhibited will readily 



r 



be understood from the curves constructed from measurements of 



