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TEXTBOOK OF BOTANY 



esting subject of research. In spite of the fact that some of the conchi- 

 sions of today may have to be modified as additional facts are discovered, 

 some of them are too interesting to be omitted even from a general text- 

 book of botany. 



Cell enlargement in stems appears to be dependent upon hormones 

 formed in young leaves exposed to light, though an excessive amount 

 of these hormones inhibits cell enlargement. In contrast to the cells of 

 stems, the enlargement of cells in the growing tips of roots is inhibited 



Fig. 73. Geotropic growth in a seedling after it was clamped in a horizontal position. 



by these hormones except when only the merest trace of them is present. 

 They seem to affect some process involved in the extension of cell walls. 

 Their movement down the stem or up the root may be deflected by such 

 external factors as light and gravity. In horizontal stems and roots they 

 are deflected toward the force of gravity. Consequently, if a seedling is 

 placed horizontally they accumulate in the cells in the lower side in 

 amounts that are favorable to the enlargement of cells of the stems but 

 inhibiting to the enlargement of cells of the roots. The stem tip therefore 

 curves upward and the root tip curves downward ( Fig. 73 ) . 



These homiones are complex organic acids that move toward the posi- 

 tive pole in an electric current. It is inferred, therefore, that the slight 

 electrical difference on the two sides of a stem or root may be the cause 



