PTERIDOPHYTA 



231 



Sporophyte. In general, the sporophyte of Selaginella has the same 

 habit as that of Lycopodium, but is nearly always smaller and more deli- 

 cate. The stems are dichotomously branched and usually trailing, but 

 are often erect or climbing. Rhizophores, which are special leafless 

 branches of the main stem, are found in many species. They produce 

 adventitious roots at their tips. The leaves are scale-like and numerous, 

 generally occurring in four longitudinal rows. Each leaf bears a ligide, a 

 minute flap-like outgrowth arising from the basal portion of the adaxial 

 surface. The ligule is prominent only 

 during the early development of the 

 leaf. 



In some species of Selaginella the 

 leaves are all alike and symmetrically 

 arranged around the stem, but in most 

 species the leaves are spread out hori- 

 zontally and usually of two kinds. 

 These are regularly arranged with ref- 

 erence to each other, there being two 

 rows of small dorsal leaves and two 

 rows of large ventral ones (Fig. 189^). 

 In contrast to Lycopodium, a definite 

 apical cell is usually present at the 

 tip of the root and stem, but some 

 species appear to have an apical meri- 

 stem (Fig. 187). 



Vascular Anatomy. The leaves have an epidermis and a loose meso- 

 phyll, the stomata usually being confined to the lower surface. The 

 chloroplasts are large and few in number, sometimes only one occurring 

 in each cell. Each leaf has a single median vein. The adult stem gen- 

 erally consists of a single, dorsiventrally flattened protostele with two 

 lateral protoxylem groups (Fig. 188). The metaxylem develops toward 

 the center, and so the stem is exarch. In some species the stele is cylin- 

 drical and in some two, three, or more separate steles are present. One 

 species has reached the siphonostelic condition. The conducting tissues 

 show an amphicribral arrangement, the xylem being completely sur- 

 rounded by the phloem. Outside the pericycle, which is generally one- 

 layered, a broad air space occurs. This is traversed by elongated cells 

 (trabeculae) consisting of endodermal cells united with one or more cells 

 that have a common origin with the endodermis. As in Lycopodium, no 

 cambium is present and so there is no secondary thickening. In some 

 species the cortical cells are thick-walled. 



Sporangia. In all species of Selaginella definite terminal strobili are 

 present. Where the foliage leaves are all alike, the sporophylls and leaves 



Fig. 187. Median longitudinal section 

 through the stem tip of Selaginella 

 bigelovii, showing the apical cell and its 

 derivatives, X 500. 



