4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



if they are immersed in water, so that there is no advantage in pre- 

 liminary soaking. Planting of dry seeds obviates a second handling 

 of the seedlings. Under the conditions routinely employed in this 

 laboratory (growth continuously from the time of planting in red 

 light (Wratten Safelight, series o) at 25° C. and about 90 percent 

 relative humidity) the coleoptiles attain a length of 25 mm at about 

 65 hours after planting. The growth rate at this time, and for the 

 next 24 hours, is approximately 0.9 mm per hour. Plants grown on 

 agar slants in small tubes as described have shown less individual 

 variability than those handled in any other way, as on filter paper, or 

 sand, in the usual glass Avena holders, or on porous stone wicks. 

 One hundred seeds can be planted in the tubes in about 10 minutes. 



LENGTH OF COLEOPTILE 



In connection with the size of the coleoptile used for the test, three 

 factors have been studied. These are the total length of coleoptile. 

 the length of the tip decapitated, and the length of the stump used. 



The growth rate of the basal portion of the coleoptile decreases as 

 the total length of the coleoptile increases, and if 20 to 25 mm of the 

 coleoptile tip are removed, the stump makes practically no growth 

 when a plain agar block is applied. If coleoptiles are used under these 

 conditions no controls are necessary. However, the sensitivity (used 

 here as the amount of growth in excess of the control which is pro- 

 duced by application of a given amount of growth substance) of the 

 basal portion also decreases rather rapidly as the total length of the 

 coleoptile increases. The 20-mm stumps of 40-mni coleoptiles have 

 practically no residual growth under the conditions of the test, but 

 do have a rather high sensitivity. It is possible, therefore, to use 

 coleoptiles of this length without controls. More commonly, however, 

 13-mm stumps of 24- to 27-mm coleoptiles have been used because the 

 plants are ready for the test nearly a day earlier. A control set must, 

 of course, be included. 



It is not intended to suggest that it is necessary to employ plants 

 of just this length or even that the described conditions are optimal, 

 but merely to indicate the technique which has given satisfactory 

 results. As a matter of fact, since the test is essentially comparative, 

 a few millimeters variation in the length of the test plants is of no 

 consequence provided the plants are randomized throughout the 

 different sets. 



