LYCOPSIDA 87 



In Selaginella selaginoides, roots arise in regular sequence 

 from a swollen knot of tissue in the hypocotyl region, but 

 in most creeping species they arise at intervals along the 

 under side of the stem. They are simple monarch structures, 

 which branch dichotomously in planes successively at right 

 angles to each other, as they grow downwards into the soil. 

 Root-caps and root-hairs are present, just as in the roots of 

 other plants. A mycorrhizal association has been demon- 

 strated in S. selaginoides. In species with aerial branches, the 

 roots are associated with pecuhar organs, usually referred 

 to as *rhizophores', and some morphologists describe the 

 roots as borne on them, while others describe the rhizo- 

 phores as changing into roots on reaching the soil. Of these, 

 the second interpretation is probably the more accurate. 

 Rhizophores are particularly well developed in cHmbing 

 species, such as S. Willdenowii (Fig. 14G), where they grow 

 out from 'angle meristems' which occur in pairs, one above 

 and one below, at the junction of two branches. In some 

 species, only one of these is active while the other remains 

 dormant, as a small papilla. The active one grows into a 

 smooth shiny forking structure without leaves. Its branches 

 are without root-caps until they reach the soil, but then root- 

 caps appear and all subsequent branches take on the appear- 

 ance of typical roots. This is the normal behaviour, but the 

 fate of the angle meristems appears to be under the influence 

 of auxin concentrations, for damage to the adjacent branches 

 may result in their giving rise to leafy shoots, instead of 

 rhizophores. It is clear that the rhizophore fits neither into 

 the category 'stem' nor into the category 'root', but exhibits 

 some of the characters of each. It is not surprising, therefore, 

 that in the days when botanists believed in the reality of 

 these morphological categories, the rhizophores of Selagin- 

 ella were the subject of much argument. 



The stem apex shows an interesting range of organiza- 

 tion, from species to species, for those with spirally arranged 

 leaves tend to have a group of initial cells, while dorsiventral 



