390 



PART IV. CLASSIFICATION. 



THE SPOROPHYTE. The primary stem is slender and elongated, erect, or 

 more commonly procumbent ; its symmetry is bilateral, isobilateral when 

 erect, dorsiventral when procumbent ; the branches spring from the flanks 

 of the primary stem, and, as this is subsequently repeated, the resulting 

 branch-system lies in one plane ; the mode of branching is lateral, though 

 it appears to be dichotomous. In some few species, however, the branches 

 have radial symmetry (e.g. 8. spinosa). 



At the points at which the normal branching takes place, leafless 

 branches, termed rhizopkores, are in some species developed in a plane at 

 right angles to that of the normal branching ; thus in S. Kraussiana 

 they arise singly on the upper surface of the stem at the points of normal 

 branching, whilst in S. Martensii two are developed at each such point, 

 one on the upper and one on the lower surface. The direction of growth 

 of the rhizophore is such that the apex eventually penetrates into the 

 soil, when roots arise from it and it ceases to grow. 



These organs have been regarded as roots, and are sometimes so desig- 

 nated still. But in view of the important morphological facts that the 

 rhizophore is of exogenous origin like 

 the leafy branches; that it has no 

 root-cap, whilst the true roots of Sel- 

 aginella have one ; and finally, that 

 sometimes a rhizophore will develope 

 leaves and even cones, the probability 

 is that they are modified branches com- 

 parable with the root-like branches of 

 the Psilotacese. 



The leaves can be readily distin- 

 guished as either foliage-leaves or 

 sporophylls. A characteristic feature 

 in their morphology is the development 

 of a small lirjuh on the upper surface 

 of each leaf near its base. 



The foliage-leaves are simple, small, 

 sessile, and rather numerous. Those 

 borne on the radial branches are all 

 alike, and are arranged spirally; the 



bilateral branches show remarkable heterophylly, there being leaves 

 of two sizes in decussate pairs, each pair consisting of one lai-ge and one 

 small leaf ; when the branch bearing these two kinds of leaves is dorsiven- 

 tral, the four rows of leaves show displacement, with the result that the 

 two rows of small leaves are borne on the upper surface of the branch, and 

 the two rows of larger leaves are borne infero-laterally (Fig. 232). 



The sporophylls are generally collected into more or less distinct cone- 

 like flowers ; they do not differ materially from the foliage-leaves, and, 

 like them, may be all of one size or of two sizes. 



The sporangia are shortly stalked and unilocular ; they arise singly 

 from a group of superficial cells of the stem just above the insertion of 

 fach sporophyll ; the wall, when mature, consists of two layers of cells; 



FIG. 2S2.-Sel<iginella helvetica (nat. 

 size) : s the upright fertile shoot, with 

 sporangia in the axils of the leaves. 

 On the procumbent sterile shoot, the 

 leaves on the under side i) are larger 

 than those on the upper side (o). 



