THE XOOT. 67 



always temporary, disappearing as the embryo becomes 

 larger. It is sometimes wanting from the first. In both 

 fernworts and seed-plants the primary root is rarely wanting, 

 but often short-lived, dying after the plant has established 

 itself and has formed secondary roots to take its place. In 

 many cases, however, the primary root persists throughout 

 the life of the plant. 



76. Secondary roots. Secondary roots, on the contrary, 

 are those which arise upon stem or leaf, or even upon the 

 primary root itself. In the last case they are distinguished 

 from branches of the primary root, which arise in regular 

 succession toward the apex, by originating out of this regular 

 order. Secondary roots are also called adventitious roots. 

 They may take their origin at any point upon any of the 

 members. Their point of origin will depend largely upon 

 external conditions. They are especially likely to be formed 

 upon those parts which are in contact with the substratum, 

 or from those parts which are kept moist. Upon stems they 

 are most apt to appear near the nodes. (See ^[ 119.) If 

 the plant as a whole is surrounded by very moist air, roots 

 may appear at any point of the surface. Secondary roots 

 arising thus upon a part of the plant exposed to the air, and 

 growing for all or part of their existence in the air, are also 

 called aerial roots. Familiar examples are to be seen about 

 the lower part of the stem of Indian corn, the English ivy, 

 the poison-oak, the trunks of palms and tree-ferns. Secon- 

 dary roots often arise in regular succession toward the grow- 

 ing apex of the stem, particularly in plants which have creep- 

 ing or subterranean stems. 



77. Growing point. --Primary and secondary roots do 

 not differ materially in their structure. The early divisions 

 of the quadrant cell which produces the primary root in fern- 

 worts are so arranged that a cell shaped like a four-sided 

 pyramid is produced. This cell becomes the apical, or 



