Growth Substances 407 



direct effect not only on the embryo but on the tissues of the ovary 

 itself, tending to make these somewhat resemble ones of the paternal 

 parent. This metaxenia must evidently be due to gene-produced chemi- 

 cal factors introduced through the pollen tube and modifying the de- 

 velopment of such maternal tissues as the pericarp and the seed coat. 



The development of galls with specific external and internal struc- 

 tures (p. 285) produced by fungi or insects seems to be dependent on 

 various chemical substances. In insect galls these may be injected into 

 the plant by the insect but more probably they result from secretions 

 from the growing larva. 



In other galls, particularly the one most actively studied— crown 

 gall— auxin is clearly involved (p. 294). Experiments with tissue cultures 

 have shown that cells of normal tissue in many cases are unable to 

 grow unless supplied with auxin. Cells of bacteria-free crown-gall tissue, 

 however, can do so. This fact suggests that the change from normal to 

 tumor tissue may result from the acquirement by tumor cells of the 

 ability to synthesize auxin. It is probable, however, that the problem is 

 more complex and that changes in the ability to form other growth 

 substances are also involved. Thus Braun and Naf ( 1954 ) have extracted 

 from crown gall a biologically active substance which is not auxin but 

 which, in association with auxin, produces active proliferation of to- 

 bacco-pith tissue in culture. Neither this nor auxin alone has a growth- 

 stimulating effect of this sort. The question of the relation of auxin to 

 crown-gall formation has been actively investigated. The subject has been 

 reviewed by Braun and Stonier (1958). 



Growth substances are also involved in the production of other gall- 

 like structures. Swellings and malformations somewhat resembling typical 

 root nodules associated with nitrogen-fixing bacteria have been induced 

 by application of synthetic growth substance on the roots of several types 

 of leguminous plants (Allen, Allen, and Newman, 1953). 



A number of other growth substances deserve mention here. Adenine, 

 for example, has been found to possess significant properties, especially 

 for leaf growth, and the balance between it and auxin seems to determine 

 the character of development in some cases (p. 414, and Skoog and 

 Tsui, 1951). 



The synthetic growth substances are too numerous to be discussed 

 here. Of particular note is 2,4-dichlorophenoxyacetic acid (2,4-D), im- 

 portant because of its wide use as a herbicide. It produces such profound 

 growth abnormalities that death usually ensues (Kaufmann, 1955; Fig. 

 18-24). For some unknown reason it has relatively little effect on 

 monocotyledonous plants. Work with it has been reviewed by Wood- 

 ford, Holly, and McCready (1958). 



Maleic hydrazide is important in that it inhibits growth in a wide 



