690 LIGHT AND LIFE 



room, and the light-grown sections were exposed to ca. 1200 foot- 

 candles of fluorescent light. All sections were incubated for ca. 20 

 hours and measured to the nearest 0.1 mm under a dissecting micro- 

 scope equipped with an ocular micrometer. Sections initially longer 

 than 5 mm were measured to the nearest 0.5 mm with a metric scale 

 ruler. Fresh weights of the sections before and after growth were 

 obtained to the nearest mg on a Roller-Smith torsion balance. The 

 basic growth medium for both green and etiolated sections con- 

 sisted of 0.01 M pW 6.1 KH0PO4 — NaoHPO^ buffer, 1-2% sucrose 

 luiless otherwise mentioned, and indole acetic acid (lAA) and gib- 

 berellic acid (GA) wherever mentioned in the text. In general, etio- 

 lated sections were supplied with lO-*' M lAA and green sections with 

 10-^ M lAA. Both types of sections received 10-^ M GA. 



Experimental Results 

 A. Calibration of the Growth System 



Effect of section length on growth. A meaningful comparison of 

 the growth of green and etiolated sections can be made only if op- 

 timally growing sections of both types of tissue are employed. This 

 required a determination of the optimal length of section to be used 

 in these experiments. To our knowledge, no such determinations have 

 previously been made, despite a multiplicity of investigations on pea 

 sections. Figs. 2 and 3 represent the percentage increase in length 

 and fresh weight of both green and etiolated sections as a fimctioii 

 of initial length and fresh weight of sections. It is clear that sections 

 .8-5 mm long are close to optimal for both types of sections, with 

 respect to both kinds of measurements. Longer sections of the etio- 

 lated sections rapidly decline in efficiency of growth, while longer 

 sections of the green material remain close to maximally efficient. 

 This resembles the situation in the intact plant, and probably reflects 

 the fact that cell division is restricted to the apical region in etiolated 

 peas, but is more distributed over the internode in green peas. Since 

 many previous experiments had been performed with 5 nmi sections, 

 which are here shown to be close to maximally effective growth, this 

 length was adopted as a standard. 



Effect of bud configuration on groivth. In both etiolated and green 

 jiea stems, the terminal bud may assimie a variety of forms (Fig. 4) . 

 In etiolated plants, the apex of the stem may be recurved (A) , hooked 

 (R) , or open (C) . In the green plants, the terminal bud may be com- 

 pletely enclosed within the stipules (A) , barely extruded (R) , or 



