l6o PLANT GROWTH SUBSTANCES 



and only a slight effect on the endodermis. Xylem, phloem, and rays 

 proliferated, but no roots developed. Naphthoxyacetic acid (2) appUed 

 to tomato stems produced responses similar to those from indoleacetic 

 acid. Root primordia resulted from the activity of the pericycle and 

 phloem parenchyma, the root cap arising from the endodermis. 



The next important group of compounds to be investigated was the 

 substituted phenoxy group. These compounds attracted great interest 

 because of their marked formative and telomorphic effects, as well as 

 the fact that many plants treated with them are killed. Numerous 

 investigations on the reactions of plants to them led to their use as 

 selective herbicides. Recently some of them have been employed for 

 several other important purposes. 



The first report dealing with the histological responses induced by 

 any of these substances was a rather brief description of the histological 

 changes in bindweed and sow thistle following applications of 2,4- 

 dichlorophenoxyacetic acid (2,4-D) in herbicidal concentrations (37). 

 Following this there were three papers (5,6,36) deahng with the reactions 

 of the bean to these compounds, especially 2,4-D and 2,4,5-trichloro- 

 phenoxyacetic acid (2,4,5-T). In two of these reports (5,6) both lanolin 

 and Carbowax 1500 had been used as carriers of the growth substances, 

 and it was found that the carrier played a role in the effectiveness of 

 some of the substances. In general the tissue responses were similar 

 in kind but greater in degree than those induced by indoleacetic acid 

 and several of the other substances. Endodermis, cambium, phloem, and 

 ray parenchyma were strongly activated, and in most instances roots 

 developed abundantly, mainly from the phloem and ray parenchyma. 



Somewhat similar experiments were done on decapitated bean plants, 

 using 2,4-D and four of its salts, namely, ammonium, copper, calcium, 

 and magnesium 2,4-dichlorophenoxyacetates (30). Histological responses 

 in the early stages showed considerable similarity. Later certain dif- 

 ferences became evident, but no response induced by one substance was 

 entirely absent in the reaction to another substance, since all responses 

 appeared to fall within the range of effects distinctive for 2,4-D. 



A 2 per cent phenylacetic acid and lanolin mixture (38) applied to 

 the cut surface of decapitated young bean plants produced flat-topped, 

 somewhat tuberculate tumors. The bulk of the tumor arose from a 

 marked proliferation of the inner cortex, endodermis, and primary 

 phloem parenchyma. The derivatives matured as tracheids or paren- 



