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RELATIONSHIPS OF SYMMETRY 



FIG. 5*. Selaginella 

 sanguinolenta. Apex of 

 an ordinary isophyllous 

 shoot. There are four 

 rows of equally large 

 leaves of similar con- 

 figuration. 



FIG. 59. Selaginella sanguino- 

 lenta. Dorsiventral anisophyl- 

 lous shoot seen from above. 

 Magnified somewhat more highly 

 than is Fig. 58. 



all of equal size and are inserted transversely to the long axis of the 

 shoot (Fig. 58). In addition to these shoots there are others exhibiting 

 anisophylly although not in so high a degree as it appears in other species 



of Selaginella. The leaves of the 

 upper side of these shoots are 

 smaller than are the lateral leaves 

 (Fig. 59), and they have an oblique 

 insertion which places them in 

 a favourable position to incidence 

 of light without their overlapping 

 one another to any considerable 

 extent. Shoots possessing such 

 leaves grow in shaded and moist 

 situations, and the leaves are 

 larger than they are in the 

 isophyllous shoots as the habitat 

 would lead us to expect. They 

 have become plagiotropous and slightly dorsiventral under the influence 

 of feeble unilateral illumination. 



The dorsiventral structure which in this species is a direct consequence 

 of external factors is in the other species of Selaginella, so far as we know, 

 inherited, and is independent of external factors. In many species, for 



example Selaginella caulescens, the 

 shoots are at first orthotropous and 

 completely isophyllous, but subse- 

 quently become plagiotropous and 

 anisophyllous ; whilst in others the 

 anisophylly appears from the beginning. 

 The lateral leaves turn their morpho- 

 logically upper surface to the light, those 

 which stand upon the upper side have 

 their morphologically under surface 

 directed to the light. As a consequence 

 of this the four rows of leaves are 

 characteristically placed so that they 

 do not intersect at right angles (Fig. 60). 

 This happens in other anisophyllous 

 plants, as we shall see, and it is an 

 arrangement by which the leaves which 

 are chiefly concerned in assimilation are brought into a favourable horizontal 

 direction. The dorsiventrality is expressed in the vegetative point, for 

 its shaded side is flattened and its outline on cross-section is not circular, 

 but elliptic. In many species of Selaginella asymmetry of single leaves 



FlG. 60. Selaginella haematodes. Portion of 

 a shoot seen from above. The leaves upon the 

 upper side are smaller than those upon the under 

 side ; each leaf also has unequal sides. 



