The origin and development of the root in the isophyllous Selaginella wallacei and the anisophyllous S. kraussiana have been investigated, and the observations agree closely with those previously made by the authors on the isophyllous S. densa. In both species the root arises from an angle-meristem on the upper surface of the stem at the angle of each unequal terminal branching of the shoot. In its early development, the root meristem is not covered by a cap, but after a small amount of growth, a root cap is initiated as a result of the formation of distal segments by the apical cell and its immediate derivatives. Soon after cap formation has begun the root meristem undergoes terminal branching beneath the root cap, but there is a considerable delay before branching of the root is visible externally. In both of the species investigated, as previously in S. densa, no evidence for a rhizophore or for endogenous root origin could be found. These observations are discussed in the light of reports by previous workers in which similar structures are interpreted as rhizophores bearing endogenous roots.

No abstract

The antiquity and diversity of Selaginella species attest to the evolutionary success of the genus. This success may be attributed, in part, to certain morphological and anatomical features that characterize Selaginella. Recent developmental studies of anisophylly, monoplastidy, the ligule, the rhizophore, marginal warty cells, and marginal teeth of the leaves, as well as aspects of the heterosporous life cycle are discussed. Unresolved questions concernmg vegetative features include the function of the ligule, morphological interpretation of the so-called rhizophore, and a possible role of distinctive leaf ornamentations In water economy. Among unresolved questions concerning reproduc ive morphology are the basis of heterospory, control of sporangial development, spore dispersal mechanisms as they anec inbreeding and outcrossing, and other details of the reproductive process. It is concluded that the genus belagine offers numerous opportunities for future research.From the standpoint of both development and Selaginella are far from obvious. It is my contenevolution, the genus Selaginella raises a number tion that, at least in part, the reasons may be foun of intriguing questions. In this regard the following quotation from Bierhorst (1971) seems appropriate: unique and reproductive features. These include amsophylly, the ligule, the presence of certain foliar â"¢ f ., c 1 • II â-I J c 7 • rf J epidermal ornamentations, the aerial root (rhizo-Ihe lamiiy belagmellaceae mcludes belagineila and , v i i j * *^^t^ of the ^eral very closelv related fossil forms which are known P^ore), monoplastidy, and certam aspeciĥ eterosporous Hfe cycle (Fig. 1). It should be em-Selaginella is probably one of the oldest of all extant phasized that although each of these features may genera of vascular plants, second only to Lycopodium, ^^ f^^^^ -^ ^^j^^^. j^^^^ groups (for example, the Despite its great antiquity, which might lead one to v i • r j • r . n oc in Spladnella), expect specialization and relative genetic stagnation, ^^g^'^ '' ^^^^^ ^^ ^^^^^^^ ^' ""^^ ^^ ^ !Lne to several very closely related fossil forms which are known from Lower Carboniferous and more recent strata.' The author thanks M. J. Spring for preparing the illustrations in Figure 1. ,,.o ^043

The development of dorsal and ventral angle-meristems in excised Y-shaped stem segments of Selaginella martensii grown on filter paper moistened with distilled water has been investigated. Based on the position of the branchings relative to the shoot tip on the intact plant, segments were designated as young, intermediate, or old. In most segments, with no auxin treatment, ventral angle-meristems formed shoots. In most young segments the dorsal angle-meristem formed a root, whereas most old segments formed a dorsal shoot. A developmental study suggests that (1) in young segments early development of a ventral shoot, controls development of the dorsal angle-meristem as a root, and (2) in old segments where both angle-meristems develop at approximately the same time, there is no ventral control over dorsal development, and a dorsal shoot results. Experiments with externally applied auxin suggest that auxin is an effective factor in controlling ventral angle-meristem development as a root.

The origin and developinent of the root of Sela,oi?~ella m a~t e n s i i have been investigated. The roots arise from angle-meristems s i t~~a t e d a t each branching of the stem. A ventral angle-meristem is present a t each forking, and a t some for1;ing-s a corresponding dorsal angle-meristem is also present. As the root grows through its aerial length (several centimeters), the tip undergoes bifurcation and a root cap is lacking. When the tip nears the soil, a cap is formed by distal segmentation of cells in the apical meristem. Externally visible branching and root hair formation u s~~a l l y occur soon after a cap is formed. The observations of root developn~ent in S. martensii agree in general with observations previously made on S . dense, S . wallacei, and S . krazlssiana. No evidence for a "rhizophore" or endogenous root initiation a s described previously by others has been found.The r e s~~l t s are discussed in the light of the concept of a "rhizophore" 111 Selaginella.