macdougal: mechanism and conditions of growth 9 



such a degree as to have solved most of the mechanical difficulties 

 so far as apparatus was concerned. The results presented in the 

 present paper were obtained by the use of a horizontal microscope 

 and by a number of auxanometers of the type described in 1901.^ 

 Various types of levers and supports were made to fit special 

 preparations of plants (plate i). 



Nearly all studies of growth have been made on seedlings or 

 on the tender and slender parts of young shoots, or on minute 

 structures such as the sporangiophores of fungi in which it had 

 been practically impossible to make any reliable determinations 

 of the constituency or physical condition of the growing structure, 

 or of its direct reactions. 



The choice of material therefore assumed a major importance 

 in any possible advance that might be made in an investigation of 

 the matter. A review of the plants available at the Desert 

 Laboratory made it apparent that the platyopuntias offered 

 certain features by reason of which it might be possible to secure 

 measurements capable of correction and analysis to a degree not 

 attainable with any other material. The flattened mature joints 

 of these plants have an elongated oval outline with a length of 

 10 to 20 cm., a width of 8 to 20 cm. and a thickness of i to 3 cm. 



Anew joint first emerges as a bud thickly sheathed with ephem- 

 eral leaves from one of the distal areolae of an old joint, in 

 March to May in the Tucson vicinity. By the time a length of 

 2 cm. is reached, both lateral and longitudinal expansion assume 

 proportions which are maintained until maturity is reached and 

 it is at this time that it is profitable to bring the apex of the bud 

 in bearing upon the lever of an auxanometer. The alterations 

 in length and width which ordinarily ensue may be illustrated 

 by the following measurements made with a ruler on a new joint 

 arising from an old one into which the bulb of a mercurial ther- 

 mometer had been thrust. 



1 MacDougal, D. T. Practical textbook of plant physiology, p. 291. 1901. New 

 York. 



2 Leaves cut off. 



