304 



A TEXT-BOOK OF BOTANY. 



layers. As indicated, gravity also determines the upward perpen- 

 dicular direction of the shoot, which is therefore said to be 

 NEGATIVELY GEOTROPic, but, as in the case of the root, the branch'es 

 are less influenced by it and hence diverge at various angles from 

 the main axis. 



Some of the other effects of gravity may be noted. If the end 

 of a shoot be cut off, the branches next to the top will grow per- 



FIG. 165. Mangrove forest (Rhizophora Mangle}, showing the habit of growth, es- 

 pecially the numerous aerial roots which form an almost impenetrable thicket. The man- 

 grove is common along the southern shores of Florida, in the Bahama Islands, and in the 

 West Indies. Many shellfish, lobsters,- and other forms of sea life are often found clinging 

 or attached far up on the roots where they become lodged during high tides. Photograph 

 from article by Henry Trimble on Mangrove Tannin in Contributions from the Botanical 

 Laboratory of the University of Pennsylvania, 1892, p. 50. 



pendicularly upward and thus assume the work of the main axis. 

 Likewise in the case of roots, if the apex of the main or tap root 

 be cut off, the branches near the end will assume a perpendicular 

 direction. It will frequently be noticed in the case of trees which 

 have been uprooted or where branches have been bent over hori- 

 zontally that the new branches which arise grow perpendicularly 

 upward. Creeping shoots furnish another good example showing 



