July 29, 1920J 



NATURE 



683 



distal sporangia, which represent their trans- 

 formed tips. Sometimes the sporangia were them- 

 selves forked. The protocorm was bedded in the 

 peat, and parenchymatous, with many rhizoids 

 (Fig. i). The cylindrical stems stood upright 

 from it, and were about 2 mm. in diameter. They 

 were traversed by a simple stele with a solid core 

 of tracheides, surrounded by phloem. The stele 

 forked at the dichotomies of the stem, but stopped 

 short at the base of the sterile columella, which 

 ran upwards into the flat-topped, and apparently 

 indehiscent, sporangium. The latter appears as 

 a transformation of the end of the stalk, which 

 is simply an ordinary branch of the plant. The 

 spores are tetrahedral, as they are in all of these 

 plants of the chert. The general aspect of Hornea 

 is such as to provoke comparison with the Bryo- 

 phytes, notwithstanding certain strongly diver- 

 gent characters. This may have some real sig- 



thing in the nature of appendages. The upright 

 stems bifurcate as before, bearing distal sporangia 

 similar to, but smaller than, those of R. major. 

 But near to their base there are "hemispherical 

 projections," apparently of superficial origin. > 

 Some of these gave origin to tufts of hair, but 

 others produced adventitious branches, which, 

 having narrow bases, were easily detached, and 

 served as means of vegetative propagation. 

 Though these organs are not easily ranked with 

 those of living plants, they are something in ad- 

 vance of what is seen in Hornea and R. major. 

 The sporangia are relatively small, and there is 

 no clear evidence of their dehiscence. 



The largest, as it is also the most complex, of 



j these plants is Asteroxyloii Mackici. Its base 



f/ioia] [Dr. Kids ton. 



Fig. 2. — Aerial stem o^ Rhyni.t major s^tn in transverse section (X20). 



nificance in view of its small size, and relatively 

 simple structure. 



Rhynia major is larger and better preserved, 

 but still it also is structurally simple. It had a 

 less distended rhizome, from which the robust 

 cylindrical stems arose. These consisted, as in 

 Hornea, of a central stele with solid xylem-core 

 and investing phloem, surrounded by a massive 

 cortex, of which the inner region appears to have 

 been photosynthetic. Outside was a well-marked 

 epidermis with stomata. These and the vascular 

 tissue prove the aerial habit of the plant (Fig. 2). 

 The stems ended in solitary massive sporangia, as 

 much as 12 mm. in length, without a columella, 

 and filled with tetrahedral spores (Fig. 3). 



Neither of the species described bore any appen- 

 dages on its stems. But Rhynia Givynnc- 

 Vaughani, though smaller than R. major, shows a 

 feature of morphological advance towards some- 

 NO. 2648, VOL. 105] 



Photo] l'^'- '^'a'ston. 



Fig. 3.— Sporangium of Rhynia ;«ayV»- filled with spores (X5J). 



consisted of branched rhizomes, which burrowed 

 after the manner of Stigmarian rootlets, and each 

 was traversed by a vascular strand with undiffer- 

 entiated xylem ; but curiously enough rhizoids are 

 absent. These rhizomes passed over into upright 

 aerial stems, which attained a diameter of as 

 much as a centimetre, and had a complex struc- 

 ture. They forked, and bore externally small and 

 simple leaves. The stele had a stellate xylem 

 very like some Lycopods. From its rays issued 

 strands passing to the bases of the leaves, but not 

 entering them. As in Lycopodium, more than one 

 vertical series of leaf-traces may issue from each 



