l68 PLANT GROWTH SUBSTANCES 



dicotyledonous plants. They were sprayed with .01 per cent aqueous 

 solution of 2,4-D. 



All the plants were in a vegetative stage when treated. Samples 

 of the stem of monocotyledonous plants were taken in the region that 

 showed a response to the growth substance, such as swelling of the stems 

 or the presence of lateral roots. The dicotyledonous plants were sampled 

 at the second through the eighth internode from the stem tip. Prolifera- 

 tion of cells was apparent in some plants soon after treatment, while in 

 others several days elapsed before a response was evident. The dicotyle- 

 donous plants usually made a greater anatomical response to 2,4-D than 

 did the monocotyledonous plants. 



Tomato plants were sensitive to this growth regulator. Eighteen days 

 after treatment lateral roots had emerged through the cortex. The cells 

 of the phloem parenchyma, cambium, ray parenchyma, and pericycle 

 proliferated to form root primordia. Proliferation was not limited to the 

 interfasicular region as it was in plants such as cocklebur, for bands 

 of active cells encircled the outer stelar region (Fig. 1,2). 



Fourteen days after treatment with 2,4-D cell division had occurred 

 in some of the cortical cells of Peperomia; but the cells most sensitive 

 to this treatment were those adjacent to the vascular bundles, par- 



LEGEND FOR FIGURES i TO 13 



Figure I. Fourth internode of tomato control plant. Figure 2. Fourth 

 internode of tomato eighteen days after treatment with 2,4-D; band of lateral 

 roots formed around stem. Figure 3. Fourth internode of control plant of 

 Peperomia. Figure 4. Treated stems oi Peperomia responded to 2,4-D; prolifera- 

 tion of cells and adventitious root formation was apparent. Figure 5. Second 

 internode of cocklebur 97 hours after treatment with 2,4-D showing pro- 

 liferation of cells in the interfasicular region of the stem. Figure 6. Eighth 

 internode of cocklebur 25 days after treatment; lateral roots emerging through 

 the cortex from the interfasicular regions. Figure 7. Cross section of a leaf 

 25 days after treatment with 2,4-D; leaf swollen and root primordia dif- 

 ferentiating. Figure 8. Stem tip of cocklebur 25 days after treatment with 

 2,4-D; cells of leaves surrounding stem tip have proliferated, stem tip has 

 broadened, and cells below apex have become necrotic. Figure 9. Stem tip of 

 normal plant. Figure 10. Fourth internode of Coleus 35 days after treatment; 

 root primordia formed from fasicular region of stem. Figure 11. Fourth 

 internode of untreated stem of Coleus. Figure 12. Untreated plant oi Drucena 

 showing cambial region. Figure 13. Lateral roots from stem of Dracena 24 

 days after treatment with 2,4-D. 



