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TEXTBOOK OF BOTANY 



( Fig. 68 ) . They are also influenced by such external factors as water, 

 oxygen, temperature, and light. 



Perhaps with further research, stems of nearly any plant may be used 

 successfully as cuttings. In addition to the factors mentioned above, the 

 origin of roots in stem cuttings varies with the age of the plant, with con- 

 ditions under which it grew, with the part of the stem taken as a cutting, 

 with the season of the year, and with the presence of special chemical 

 substances. 



Experiments have shown that some thirty different chemical com- 

 pounds, when applied artificially, accelerate and increase the initiation 

 of root primordia in parenchyma cells in almost any part of the plant 

 ( Fig. 192 ) . Such compounds are of value in shortening the time of root 



Fig. 192. A-B, effect of treatment of African marigold plants with 1 per cent 

 carbon monoxide for 10 days: A, untreated; B, treated. Numerous roots have 

 developed from the stems and petioles. C, adventitious roots on stem and petioles 

 of African marigold 6 days after a 3-day treatment with 0.5 per cent acetylene. 

 Photo by P. W. Zimmerman and A. E. Hitchcock, Boyce Thompson Institute. 



