IMPORTANT MOVEMENTS OF PLANTS III 



By John J. Birch 



GEOTROPISM AND APOGEOTROPISM. 



Geotropism is the downward movement of sub-aerial 

 organs. All roots penetrating the ground are guidedi in their 

 downward course by this factor. The manner in which it 

 acts seems quite clearly understood. Gravitation draws the 

 heavier content of the cells, especially the starch grains, to 

 the the bottom of the cell. Here their accumulated weight 

 jiresses on the sensitive protoplasm and produces a condition 

 of strain, differing from anything in the upper part of the 

 cell, and it is this difference which establishes the line of di- 

 rection. When the responding mechanism is so set main 

 roots are sent growing towards this pressure, main stems away 

 from it and side parts across it, precisely as in other typical 

 responses. Geotropism is a ])erfect example of the fact that 

 a stimulus acts merely as a guide and not as a physcial aid to 

 responses. Gravitation might be supposed to help pull roots 

 downward, but it cannot be imagined to push stems upward 

 or to drive side parts crossways. 



The geotropism of leaves is disguised by their stron- 

 ger iieliotropism ; but that they are geotropic is shown by the 

 positions they assume when kept in dark rooms. Flowers 

 are also exceedingly geotropic. If a long terminal cluster of 

 larkspur be bent over and fastened in that position, each blos- 

 som as it opens will turn over individually to the identical po- 

 sition, it would have occupied in the vertical cluster. The 

 position of each separate flower is established geotropicalh . 



