FORM-ALTERATIONS AND GROWTH OF CACTI. 23 



The following may be cited as examples: In the year 1904-05 there were 

 heavy and frequent rains all winter, and in the latter part of April two 

 rains of over 0.5 inch. In April and May, mostly the latter, 76 intervals 

 reached their g^reatest expansion, while but 14 did so in December, Janu- 

 ary, and February. In 1905-06 the heavy rains all occurred in November, 

 and by December all rapid expansion had ceased. There were, however, 

 rains which kept the ground moist through the first part of February. In 

 the latter part of March about 0.5 inch fell, and 0.6 inch on the 8th of April. 

 Under these circumstances, while 50 intervals attained their maximum 

 before the first of March, there were 11 to do so in March and 26 between 

 April 10 and 20. The fact that 37 intervals did not attain their maximum 

 until spring was plainly not due to the lack of soil-water. In 1906-07 the 

 heaviest rains were in December and January, and after a rain of 0.6 inch 

 January 30 no more of any conseqiience fell until March 5, when there 

 was 0.25 inch precipitation, after which there was but 0.15 inch until June. 

 This year 40 intervals attained their maximum before the first of March, 

 20 in March, and 29 in the late spring and summer. 



It is impossible to study these plants through the spring, when the con- 

 ditions are so favorable to rapid shrinkage, without being convinced that 

 they do not contract as readily at that time as at other periods of the year. 

 As an example, three furrows on sahuaro No. 1 reached their maximum 

 expansion from May 3 to 5, 1905, and in spite of the fact that there was but 

 0.03 inch rain in May, only one of them contracted more than 1 unit until 

 after May 12, and they were all broader at that time than in February and 

 March, when the ground was soaked with water, while in May not only 

 was there practically no rain, but the ground was drying with the increased 

 heat. Thus, while the evidence is not absolutely conclusive, it certainly 

 points toward the conclusion that it requires a less amount of soil-water 

 to produce the same degree of expansion in warm than in cold weather. 



Reviewing all the available evidence, there seems to be no doubt that 

 both contraction and expansion as the result of changes of air-temperature 

 have been detected by the measurements of intervals on the sahuaro trunk. 

 In the majority of cases, however, this action of temperature is completely 

 masked by the greater movements which are connected with variations in 

 amount of soil-water. 



LIMITS OF EXPANSION. 



Theoretically the limit of expansion for the trunk of a sahuaro would 

 be determined by the elasticity of the outer shell after the furrows had 

 disappeared; and in some cases in the foothills, where the supply of water 

 is comparatively constant, the ridges are little more than rows of spines 

 and the outline of the circumference would show only slight irregularities. 

 In general, however, this is not true, except on the lignified bases of the 

 trunks. Where the tissues remain green the surface is always in ridges 

 and furrows of varying height, and this is true even where the ground 



