THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



581 



is not differentiated into palisade and spongy tissue, but is very open and 

 lacunar throughout. A single leaf trace passes into the leaf from the stem 

 and forms an unbranched midrib. It is con- 

 centric in structure, composed of a central 

 strand of xylem surrounded by phloem and 

 an endodermis. The leaf traces originate, 

 as in Lycopodiiim, from the protoxylem 

 group. 



Anatomy of the Rhizophore 



Each rhizophore originates from the upper 

 surface of the stem, at the point where a 

 branch is attached. It grows downwards 

 for a short time by means of an apical cell, 

 but very soon this is lost and two root apices 

 are differentiated from sub-epidermal cells 

 (Fig. 591). These roots do not at first 

 develop, but the rhizophore continues to 

 elongate by intercalary growth until the soil 

 is reached, when the roots emerge and pro- 

 duce root hairs. Subsequent root branching 

 is monopodial, but, as in the stem, looks 

 dichotomous. The lateral rootlets also 

 originate from sub-epidermal cells of 

 the main roots. They are not therefore 

 truly endogenous, as are nearly all other 

 roots. 



The vascular structures of both rhizophore and root are similar (Fig. 592). 

 There is a single concentric stele, with one protoxylem, which is central in 

 the rhizophore and at one side in the root. The root also has a good endo- 

 dermis, while that of the rhizophore is vague. The phloem surrounds the 

 xylem strand, but in the root there are no sieve tubes opposite the protoxylem. 



Fig. 



590. — Selaginella kraussiana. 

 Transparent view of the stem 

 showing bifurcation of the 

 stele at the branch nodes. 

 Leaf traces only shown at the 

 top of the figure. {After 

 Harvey Gibson.) 



The Sporangia 



The sporangia arise from a group of cells on the surface of the axis, close 

 to the base of each sporophyll.* The sporophylls closely resemble the 

 vegetative leaves in structure, and each has on its upper surface a ligule, 

 in the form of a small scale which develops from an epidermal cell close to 

 the abaxial side of the sporangium. It has a distinct foot or glossopodium, 

 which remains embedded in the tissue of the sporophyll. The ligule matures 

 before the sporangium and withers when the latter is ripe. Its cells are 



* We have retained the term sporophyll, as applied to the strobilar leaves of Seloginello, 

 in deference to general usage. The sporangia are not, however, borne on the leaves but on 

 the axis, a condition described as stachyosporous, in distinction from the condition in, 

 e.g., Lycopodium, which is truly phyllosporous. 



