macdougal: mechanism and conditions of growth 25 



acids which retard respiration and growth and lessen hydratation 

 capacity is a process which may affect the rate and amount of 

 growth in a very important manner. 



Since these various effects may be widely variant with the 

 mechanical structure and composition of the plant there is the 

 amplest confirmation of my own generalization made fourteen 

 years since that the "action of light upon growth is not invariable." 

 The most recent confirmation of this conclusion is that of Blaauw, 

 who finds important differences in the "photo-growth reactions" 

 of the sporangiophores of Phycomyces and of hypocotyls of Heli- 

 anthiis. The considerations presented in the foregoing paper 

 would make a similarity of response between two organs of such 

 unlike structure highly improbable. 



The more important suggestions, inferences, and conclusions 

 arising from the work described above may be briefly re-stated as 

 follows : 



1. The joints or segments of platyopuntias accomplish nearly all 

 their total enlargement during sixty to one hundred days of the 

 initial season in the Tucson climate. Enlargement and secondary 

 growth may take place as determined by branching and environic 

 factors, in succeeding seasons. 



2. The changes in volume of joints in the second or succeeding 

 seasons include daily reversible alterations amounting to 1/250 of 

 the total length. 



3. Reversible enlargement begins in mid-forenoon and continues 

 until afternoon, when contraction ensues and with various modifi- 

 cations may continue until the following morning, daily. 



4. Reversible daily elongations of 6 mm. were recorded and irre- 

 versible growth elongations of ten to twenty times this amount 

 were recorded. 



5. Elongation takes place during the daylight period, accom- 

 panied by decreasing acidity and lessened transpiration. The 

 maximum rate occurs about mid-day. Decrease in rate takes place 

 after mid-day, while the air temperature is still of optimal inten- 

 sity and the plant has the highest water content. 



6. Reversible changes in mature joints synchronize with growth 

 of young joints, but the extent to which the reversible changes 

 enter into or accompany growth has not yet been determined. 



7. The general features of the daily growth record suggest that 



