Meristems 



67 



influence of auxin or other growth substances (p. 386). The development 

 of axillary buds has been discussed by Sharman (1945) and by Garrison 

 (1955), who finds that they originate from a region of residual meristem. 

 After the organization of an apical meristem in the bud, the procambial 

 strands develop acropetally into the leaf primordia, as does the phloem. 

 Xylem forms at several loci and develops in both acropetal and basipetal 



directions (p. 204). 



When the shoot is producing leaves, the meristematic dome is relatively 

 low and rounded but when flower buds begin to be formed it becomes 

 steeper and more elongate. Flowers arise as modified branches, and the 

 floral parts develop from a series of primordia (Fig. 4-13). In the forma- 



<2 



Fig. 4-13. Longitudinal section through young inflorescence, showing stages in de- 

 velopment of floral primordia. i, bract; t, trace to primordium; v, procambial strand. 

 Most active meristematic areas are stippled. (From Philipson.) 



tion of more complex inflorescences, however, the meristem changes 

 markedly. Since growth in length usually ceases at this time, what is 

 formed is essentially a determinate structure rather than an indeterminate 

 one like that of the vegetative shoot. What its character will be is de- 

 cided by the size and number of the flowers and the character of the 

 inflorescence. Various factors, notably the carbohydrate-nitrogen ratio 

 and the photoperiod, determine whether the meristem forms vegetative 

 or reproductive structures (p. 184). For accounts of the development of 

 the reproductive apex, see papers by Gregoire (1938), Philipson (1948), 

 Gifford and Wetmore (1957), and others. 



The shoot meristem is not constant in size but changes during de- 

 velopment. In the young embryo it is very small, and it enlarges as the 



