NO. II ASSAY METHOD FOR GROWTH SUBSTANCES WEINTRAUB 3 



diameter that the tubes sHp in easily and stand upright ; 2.5 cm 

 between centers allows sufificient room to manipulate the tubes. The 

 length of the block, of course, determines the number of tubes and 

 will depend upon the size of the photographic paper, incubator, etc., 

 which one uses. For ready identification of the racks and the shadow- 

 graphs some suitable design (e. g., a letter or number) may be punched 

 or drilled in a strip of sheet metal which is fastened against the back 

 of the rack (see pi. i). Machine-made test tubes usually have suffi- 

 ciently uniform rims ; if the tubes are made by hand from glass tub- 

 ing it is necessary to grind the rim at right angles to the long axis of 

 the tube. A mark should be made on the tube at a given distance 

 below the rim ; if it is desired to change the depth of planting in dif- 

 ferent experiments a glass-marking pencil is convenient ; otherwise, a 

 scratch made with a file or carborundum wheel furnishes a permanent 

 mark. 



THE AGAR SLANT 



The same purified agar which is used for the test blocks is suitable. 

 The concentration of agar should not be less than 0.8 percent. Greater 

 concentrations, up to 2 percent, give equally good results ; 0.9 to 

 I percent agar has been routinely used. The agar may be made up in 

 nutrient solution if desired but tap or distilled water have given uni- 

 formly satisfactory results. The growth rate of the coleoptile will be 

 found to depend upon the composition of the agar. The simple device 

 used in bacteriological laboratories is very convenient for filling the 

 tubes with the melted agar. This consists of a funnel connected by a 

 short length of rubber tubing to a glass tip and provided with a pinch 

 clamp. The tubes are placed in the rack, filled up to the mark and the 

 whole rack is tilted backward through about 60° so that the agar 

 solidifies in a slant. The angle of the slope (and of the planted seed) 

 should be such that the coleoptile grows erect without being required 

 to curve. One hundred tubes can be charged with agar in about 8 

 minutes. 



PLANTING AND GERMINATING SEEDS 



The husked seed is pressed gently against the surface of the agar 

 slant with the groove side down and the embryo at the level of the 

 mark on the tube. The seeds may be soaked in water before plant- 

 ing or planted without previous soaking. Soaking does not affect 

 the growth rate or the sensitivity of the plants. Furthermore, husked 

 seeds planted dry on the agar absorb water nearly as rapidly as 



