178 PLANT GROWTH SUBSTANCES 



acid (Fig. 2). All of the substances having formative influences also 

 cause ovaries of tomato to develop without pollination (parthenocarpy). 

 They vary in effectiveness and practical value. For example, i mg./l. 

 of 2,4-dichlorophenoxyacetic acid (2,4-D) is as effective as 500 mg./l. of 

 some of the derivatives of benzoic acid. Though not as effective as 2,4-D, 

 a- (2-chlorophenoxy) propionic acid is recommended for practice because 

 it does not modify leaves. 



Physiological activity of a specific type and for a specific species is 

 associated with molecular configuration as a whole rather than any one 

 part of the molecule. A few examples may be given: 2,4-D is one of the 

 most active compounds known for many hormone-like responses, but 

 2,6-D is practically inactive; 2,4-D and 4-chlorophenoxyacetic acid both 

 have power to elongate cells and modify the leaves and other organs of 

 tomato {Lycopersicon esculentum Mill.) plants. The latter compound 

 causes a striking modification of Turkish tobacco {Nicotiana tabacum L.) 

 leaves and Kalanchoe daigremontiana Hamet et Perrier plants while 2,4-D 

 does not. 2-Chlorophenoxyacetic acid and 4-chlorophenoxyacetic acid 

 cause cell elongation and modification of tomato leaves while the 

 propionic and butyric homologs cause cell elongation but not modifica- 

 tion of leaves. 2,4,5-Trichlorophenoxyacetic acid and higher homologs 

 cause cell elongation but not modification of leaves; 2,4,6-trichloro- 

 phenoxyacetic acid does not induce cell elongation but modifies leaves. 

 More illustrations of this sort are shown in the accompanying tables. 



Physiological activity can be determined at the present time only by 

 biological tests. Both molecular configuration and the genetic constitu- 

 tion of the species are involved. Tomato and tobacco are closely-related 

 species, but 2,4-D modifies only tomato, whereas 4-chlorophenoxyacetic 

 acid modifies both. The mechanism of modification in the plant is com- 

 plex, and we can only theorize on what combination of factors makes for 

 activity or inactivity. 



The recent work of Burton (3) is welcomed as one of the first attempts 

 to determine what happens to the structure of tissue to bring about these 

 odd forms. Using the bean leaflet as a test object Burton worked out the 

 normal structural developments and compared these with chemically 

 induced modifications. It appears that the normal bean leaflet develops a 

 lamina by the activity of a subepidermal marginal meristem, which 

 produced four internal layers of plate meristem. The adaxial (upper) 

 of these layers develops into the palisade layer and the other three 



