398 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



Although the results of auxin treatment in rooting of cuttings are 

 in general very striking, there are some plants which do not respond 

 markedly even to this. Recently we have studied some of these so- 

 called "diflBcult" plants. It appears that some of them, such as 

 Canadian hemlock and blue spruce, may be readily rooted if care 

 is used and auxin in the right concentration is applied (pi. 1). Some 

 others, such as white pine and Norway spruce, can also be rooted, 

 but only when the plants from which cuttings are taken are them- 

 selves young (pi. 2) . It is important to note that it is not the age 

 of the cutting which is important, but the age of the tree from which 

 it is taken. Occasionally, too, other substances, such as sugar, vitamin 

 Bi, etc., when used together with auxin, promote the formation of 

 roots. 



FiGiTBE 4. — Inward curvature of slit stems in auxin solutions. Left to right: 

 water, 0.2, 1, and 5 mg. auxin per liter. Photographed after about 30 hours. 



Another fact of some importance in the rooting of woody cuttings is 

 the type of shoot used. In some of the conifers it is clear that there 

 is a difference in response between the side shoots (laterals) and the 

 apical or terminal shoot. The latter, even if supplied with sufficient 

 auxin, roots less readily and dies more quickly than the side shoots. 

 Thus here again the response to auxin varies with the part of the 

 plant. 



Perhaps the most remarkable variation within the plant is shown by 

 the different responses of different parts within the same section of 

 stem. If stems of young pea plants are slit in two and placed in auxin 

 solution, the two halves curl inward toward each other. The extent 

 of the curvature varies with the concentration of the solution, and 

 the reaction can thus be used as a test for the auxins (fig. 4) . Many 

 other plants show the same phenomenon; dandelion stalks are very 

 responsive. The development of the curvature can best be shown 

 in the form of a movie, taken by lapse-time photography. The halves 



