FORM-ALTERATIONS AND GROWTH OF CACTI. 31 



100 years. As already stated, the observations leave no room for doubt 

 that the period of active growth is in summer. Though not clearly indi- 

 cated in the table, the fact may be accepted without question. 



Plate 5 shows the characteristic change in the contour of the younger 

 plants. The earliest photograph exhibits the stage in which the circum- 

 ference increases up to within a few inches of the apex, while the later 

 pictures of the same plant show it after it has assumed its columnar form . 

 Table 8, No. 9, shows the rate of growth for the period during which the 

 photographs were taken. The pictures are not all made to the same scale, 

 as the table will show. 



BISNAGA (ECHINOCACTUS WISLIZENI). 



The measurements of Echinocactus were not so extensive as those on 

 the sahuaro, but when taken in connection with the work already done on 

 the sahuaro they serve to establish certain important facts. It is plain 

 from the beginning that the bisnaga, like the sahuaro, adjusts itself to vary- 

 ing quantities of stored water by a bellows-like action of its ribs. In the 

 winter of 1903-04 two plants were marked in the same manner as the 

 sahuaros, one with three intervals on the north and the other with three 

 intervals on the south side of the plant, and in 1909 another plant (No. 

 11 ) was marked all around the circumference, each interval including two 

 furrows measured from the edges of the ribs. Each of the three plants 

 responded in the same manner as the sahuaro to the presence or absence of 

 water in the soil, and in one case where a little water accidentally reached 

 the roots the effects were plainly evident (fig. 13), expansion replacing 

 contraction in a very dry time and continuing nearly a month from the time 

 the plant was watered. The only difference which was apparent between 

 the responses of the sahuaro and the bisnaga to soil-water seems to be that 

 contraction and expansion are somewhat less pronounced in the latter. 



In the winter of 1903-04 the greatest expansion of any interval in a bisnaga 

 was 13 units, while one interval on a sahuaro expanded 38 units, and many 

 from 15 to 20. The greatest contraction of any interval on a bisnaga was 

 16 units, and on the sahuaro the contraction reached as high as 30 units. 

 It might seem that the small number of bisnagas under observation 

 scarcely warrants this conclusion, but the same difference is also shown in 

 the single plant marked in 1909. This may be partly due to the fact that 

 in many cases a furrow of the Echinocactus represents a smaller fraction 

 of the circumference than does a furrow of the sahuaro. 



