MORPHOLOGY OF ANGIOSPERMS. 97 



NEGATIVELY GEOTROPic, but, as in the case of the root, the branches 

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

 tfie 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- 

 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 

 the influence of gravity, the branches growing upward and the 

 roots downward. 



Modified Roots. — Roots which arise from the nodes of the 

 stem or other parts of the plant are known as secondary or adventi- 

 tious roots. These include the aerial roots of the banyan tree, 

 which are for the purpose of support ; the roots of the ivy, which 

 are both for support and climbing, and the roots of Indian corn 

 and many palms which serve both for support and the absorp- 

 tion of nourishment. Under this head may also be included the 

 aerial roots of orchids and the root-like structures, or haustoria, 

 of parasites, as of mistletoe and dodder, which penetrate the 

 tissues of their host plants. 



Of special interest also are the breathing roots of certain 

 marsh-plants which serve to convey oxygen to the submerged 

 parts; and the assimilation roots of certain water-plants and 

 epiphytes, which are unique in that they produce chlorophyll. 

 In certain plants the roots give rise to adventitious shoots as in 

 Prunus, Rubus, Ailanthus, etc., and in this way these plants 

 sometimes form small groves. 



Root Tubercles. — The roots of the plants belonging to the 

 Leguminosse are characterized by the production of tubercles, 

 nodules or swellings (Fig. 64) which have been shown to have 

 a direct relation to the assimilation of nitrogen by the plants of 

 this family. Like carbon, nitrogen is one of the elements essential 

 to plant-life, being one of the constituents of protoplasm and 

 present in various nitrogenous (proteid) compounds which occur 



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