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THE WATER-BALANCE OF SUCCULENT PLANTS. 



EFFECTS OF INSOLATION. 



The same phenomenon as in the sahuaro (though again in a less marked 

 degfree), of broader furrows and thicker ribs on the north than on the 

 south side, is evident in the bisnaga; but in the bisnaga there is another 

 morpholog-ical character which is also related to insolation. The ridges 

 are not only thicker, but longer, on the north side. This causes the plant 

 to bend slightly to the south, so that the growing apex, except in the very 

 young plants, is never straight on the top, but faces the south. 



23 



27 



23 



Fig. 13. — Curve representing the sum of the variations of the intervals comprising the 

 entire circumference of a bisnaga (No. ii). Sudden rise in curve shows the combined 

 effect of a light watering and a very light shower. January 2 to May 21, igog. 



The comparative amount of response to insolation has not thus far been 

 definitely determined. On the two plants under observation in 1904 the 

 furrows on the south began to contract from one to three weeks earlier 

 than those on the north, but the fact that only two plants were being con- 

 sidered, and that one was marked only on the north and the other only on 

 the south, gives us the element of individuality to which the variations 

 might be due instead of insolation. The plant marked in 1909 showed no 

 perceptible differences in the contraction or expansion of the two sides. 



