326 CALAMITES. [CH. 



principal ray of parenchyma between each primary xylem bundle. 

 In the latter case the principal rays present the characteristic 

 appearance shown in figs. 71, 74, J., 75 and 78, a type of stem 

 which we may refer to as Calamites (Arthropitys) communis. 

 In the former case the stern presents the appearance shown in 

 fig. 83, D ^ A third variety of Arthropitys stem is one which 

 was originally named by Goppert Arthropitys bistriata ; in this 

 form the principal rays retain their individuality as bands of 

 parenchyma throughout the whole thickness of the wood^. 

 Such stems as those of figs. 73 and 74, B, may be young 

 examples of Arthropitys communis or possibly of A. bistriata. 

 The narrow secondary medullary rays of Arthropitys usually 

 consist of a single row of cells which are three to five times 

 higher than broad, as seen in tangential longitudinal section. 

 Infranodal canals occur in some examples of Arthropitys. 



In the subgenus Arthrodendron, a type of stem first 

 recognised by Williamson and named by him Galamopitys^ , 

 the principal medullary rays consist of prosenchymatous cells 

 {i.e. elongated pointed elements) and not parenchyma. These 

 elongated elements are not pitted like tracheids, and they 

 are shorter and broader than the xylem elements. In some 

 examples of this subgenus the primary rays are bridged across 

 at an early stage by the formation of secondary interfascicular 

 xylem, and in others they persist as bands of ray tissue, as in 

 Arthropitys. Other characteristics of Arthrodendron are the 

 abundance of reticulated instead of scalariform tracheids in the 

 secondary wood, and the large size of the infranodal canals. 



Fig. 83, D represents part of a transverse section of Arthro- 

 dendron: in this stem the rays have been occupied by inter- 

 fascicular xylem at a very early stage of the secondary growth. 

 The section from which fig. 83, D is drawn was described by 

 Williamson in 1871 ; the complete section shows about 80 

 carinal canals and primary xylem groups. The prosenchymatous 



^ The stem of fig. 83 is an example of Arthrodendron, but the appearance of 

 the secondary xylem agrees with that in some forms of Arthropitys. 



2 For figures of this type of stem vide Goppert (64); Gotta (50), PI. xv. 

 (specimens 13787 in the British Museum Collection); Mougeot (52), PI. v.; 

 Stur (87), pp. 27—31; Eenault (93), Pis. xliv. and xlv. etc. 



3 Williamson (71), (712), (gy), fig. 5. 



