lyo PLANT GROWTH SUBSTANXES 



quarters, pigweed, and Verbena likewise responded to 2,4-D; and the 

 tissues responsible for proliferation and root formation were the same as 

 those for the previous plants mentioned, namely the cambium, phloem 

 parenchyma, ray cells, and pericycle. 



Comparing the monocor\ledonous plants with the dicotyledonous 

 plants, the same tissues prohferated to form root primordia. Dracena 

 developed a swelling at the base of the leaves which perhaps included 

 several nodes and internodes. Upon examination of this region it was 

 apparent that a cambial-like region was present in this plant (Fig. 12). 

 Eishteen davs after treatment lateral roots ori^inatin? in the cambial 

 zone and in the parenchyma cells on either side of it (Fig. 13) were 

 extending through the cortex. 



Adventitious roots in stems of Tradescantia treated with 2,4-D were 

 formed from the meristematic zone near the periphery of the stem 

 (Fig. 14, 15). 



Philodendron failed to respond to treatment with 2,4-D. \'ascular 

 bundles were distributed throughout the stem, the larger ones occupying 

 the center. Over the phloem of each bundle was a cap of thick-walled 

 fibers which were perhaps too differentiated to revert to an active stage 

 of prohferation. .\lthough there were parenchyma cells in the stem 

 which ordinarily responded in other species they failed to do so in 

 Philodendron (Fig. 16, 17). 



.-Uthough the above ground stem of quack grass was not affected by this 

 growth substance, the rhizome made a striking response. The parenchyma 

 cells adjacent to the differentiating bundles proliferated and were re- 

 sponsible for the formation of lateral roots (Fig. 18, 19), 



The anatomical responses of plants to 2,4-D were similar regardless of 

 whether they were dicotyledonous or monocotyledonous (1,5). Pro- 

 liferation of cells and root formation from the cambium, parenchyma 

 cells, ray cells, and pericycle were common to both groups. 



From these results it has been concluded that whether a plant can 

 be stimulated generally depends on whether there are cells in the stem 

 which are immature enough to proliferate when treated with 2,4-D. 

 These types of cells were more limited in amount in monocotyledons than 

 in dicotyledons, which may explain why the monocotyledons treated 

 with 2,4-D usually recovered. 



Another growth substance which had a quite different effect on the 

 anatomical structure than 2,4-D was a-naphthaleneacetamide. Mitchell 



