18 THE WATER-BALANCE OF SUCCULENT PLANTS. 



tion, thougrh limited, is definite, and it is a matter of much interest that 

 the results of its action are seen not only in accelerating and otherwise modi- 

 fying- the mechanical adjustment of the sahuaro trunk, but are also plainly 

 reflected in its structure. 



As shown by plate 3, the furrows of the younger parts of the sahuaro 

 stem are of approximately the same width on all sides, while those of older 

 parts, including especially all the middle portion of the trunk, are dis- 

 tinctly wider on the north than on the south side. Now, as we have seen, 

 insolation, of whatever side of the trunk, results finally in marked con- 

 traction, but in this latitude, where the south side receives, for a longer 

 period of time and at a less angle of incidence, the direct rays of the sun, 

 the furrows become permanently narrower than on the north side. That 

 there is here a causal relation can hardly be doubted. The temporary 

 change which has been described, and which is only temporary as far as 

 the younger tissues are concerned, becomes permanent in the older parts 

 of the stem. 



Additional evidence of the direct effect of insolation was secured by 

 shading one of two plants growing under similar conditions. On January 

 20, 1909, a shelter was constructed for No. 10. At first this was composed 

 of a single thickness of burlap, but it did not prove a complete shade 

 either in size or thickness. On February 3 it was enlarged and an addi- 

 tional layer of burlap placed over it. 



Another plant (No. 21), about the same size as No. 10 and growing 

 within a few rods of it, was selected for comparison. No. 21 grew on the 

 same slope, was a little smaller, and not in quite as good a condition as No. 

 10, but an inspection of the two at the beginning of the experiment left the 

 impression that a comparison between them would be a fair one. 



The photographs taken in February, 1910 (plate 4), show the very evi- 

 dent difference at that time, and a comparison of the curves of the inter- 

 vals on the two plants show it even more plainly (fig. 10). The amount of 

 contraction of 8 intervals on No. 10 was 35 units and on No. 21 for the 

 same period 104.5 units. Moreover, the contraction began over a month 

 earlier on No. 21 than on No. 10; the contraction beginning March 5 on 

 No. 21 and April 14 on No. 10. 



Temperature records taken for a few days in November, 1909, showed 

 that the temperature through the middle of the day averaged 6° F. lower 

 inside the shelter than outside in the sun. 



Before leaving this division of the subject, reference may be made to an 

 interesting phenomenon apparently connected with the effects of insolation. 

 As is well known, the sahuaro produces, in early summer, a large number 

 of conspicuous white flowers very near the apex of the stem. The first 

 flower-buds, though exceptionally formed as early as March, are usually 

 seen in the latter part of April, and the plant is in full flower in May. A 

 regular order in the formation of flowers and opening of buds is observa- 



