42 THE waTer-salance of succulent plants. 



SUMMARY. 



The data presented in the preceding pages, together with the conclu- 

 sions to which they lead, may be summarized as follows: 



(1) Observations on the mechanical adjustment of the sahuaro trunk 

 have been greatly extended and the conclusions drawn from previous studies 

 have been confirmed. This has been especially emphasized in what has 

 been established regarding the direct adjustment of the plant to amounts 

 of available water in the soil. 



(2) In the course of measurements continued for a term of years it 

 has become apparent that each plant has a marked individuality as regards 

 promptness, extent, and duration of response to external stimuli, so that it 

 is even possible to predict its behavior with a considerable degree of accu- 

 racy. Parts of the same plant also show certain peculiarities of response. 

 These are referable to internal changes, and especially to shifting of pressure 

 in the tissues. The causes of the former are not yet clearly determined. 



(3) Insolation is a potent secondary factor operating in conjunction with 

 water-supply and modifying its effects, though not to such an extent as rad- 

 ically to change the curves by which these effects are expressed. Under 

 the influence of insolation, expansion after rain is more prompt, but contrac- 

 tion during long periods of drought is also more marked on the side most 

 strongly insolated. Such contraction is greater in the winter on the south 

 side of the trunk, while in summer, for the period during which there is 

 greater insolation of the north side, the reverse holds true. For the year 

 taken as a whole it results that the furrows on the south side of the sahuaro 

 are considerably narrower than those of the north side. Thus it is, seen that 

 what might be taken as a definite morphological character is, in reality, a 

 response to the action of an external stimulus, but it nevertheless becomes, 

 in the course of ontological development, a distinctive structural feature. 



(4) Apart from direct insolation, evidence has accumulated going to 

 show that slight expansion and contraction of the sahuaro trunk follow 

 changes of air- temperature, but in most cases these minor changes are 

 masked by those occasioned by variations in amount of soil-water. 



(5) After the first few years, growth in the sahuaro is apical, the incre- 

 ments of succeeding years increasing in diameter until the trunk attains 

 the full thickness, which it afterwards maintains through life. The average 

 yearly growth in height of individuals was between 10 and 12 cm. From 

 these data, which are all thus far available, it appears that a giant cactus 

 requires oU an average approximately 100 years to attain a height of 10 

 meters . Observations thus far do not establish any definite relation between 

 growth and rainfall, though it is hardly conceivable that such a relation 

 does not exist. On the other hand, the fact that growth takes place chiefly 

 in the summer time is unquestioned. 



