406 Morpho genetic Factors 



eral conclusions are that most of the changes produced are in the ab- 

 normal distribution and proportions of tissues rather than in the produc- 

 tion of new structures. Important factors in these changes are nutrition 

 and the age and state of the tissues when treated. Marked differences 

 in reaction to various substances were found, and morphogenetic 

 processes may thus in part be manipulated. A review of this work has 

 been made by Beal ( 1951 ) . 



Fig. 18-23. At left, effect of a bud grafted to the phloem region of a piece of chicory 

 root in culture. The bud stimulates the development of vascular tissue below it, pre- 

 sumably because of a growth substance it produces. At right, a similar experiment 

 except that a sheet of cellophane, CL, has been placed between the bud and the tissue 

 below. The same effect is produced, indicating that organic continuity is not neces- 

 sary. B, bud; P, phloem; V, vascular parenchyma; C, cambium; A, histologically 

 altered tissue; L, line of contact between bud and stock. (From Gautheret, after 

 Camus. ) 



OTHER FORMATIVE EFFECTS 



Various other substances of morphogenetic significance have been 

 postulated, but little is known about them. Thus in Dictyostelium, the 

 remarkable life cycle of which has been described earlier (p. 223), the 

 factor controlling the aggregation of the myxamoebae into a pseudoplas- 

 modium appears to be chemical in nature and diffusible (J. T. Bonner, 

 1949). To this substance Bonner has given the name acrasin. 



In a few traits, such as time of fruit ripening in dates ( Swingle, 1928 ) 

 and staple length in cotton (Harrison, 1931), pollen seems to have a 



