300 Royal Society : — 



ditions to these were made in lines radiating from within outwards ; 

 hence each wedge consisted of a series of radiating laminae, separated 

 by medullary rays, having a peculiar mural structure. At their 

 commencement these wedges were separated by wide cellular areas, 

 running continuously from node to node ; as the woody tissues in- 

 creased exo^'Cnously, these cellular tracts also extended outwards. 

 Radial longitudinal sections exhibited in these the same mural tissue 

 that occurs in the woody wedges. Hence the author gives to the 

 former the name of primary medullary rays, and to the latter that 

 of secondary ones. The structure of the medullary and ligneous 

 zones is compared with that of the stem of a true Exogen of the first 

 year, of which transitional form Calamites may be regarded as a per- 

 manent representative. Tangential sections of this woody zone ex- 

 hibit parallel bands of alternating vascular and cellular tissue, running 

 from node to node. At the latter points each vascular band dicho- 

 tomizes, its divergent halves meeting corresponding ones from conti- 

 guous wedges, and each two imite to form one of the corresponding 

 bands or wedges of the next adjoining internode. 



The Bark, hitherto undescribed, consists of a thick layer of 

 cellular parenchyma, undivided into separate laminse, and not exhi- 

 biting any special differentiation of parts. This structure exhibits no 

 signs of external ridges or furrows, being apparently smooth. The 

 stem was enlarged at each node, but the swelling was less due to any 

 increased thickness of the bark at these points, than to an expansion 

 of the woody layer at these points, both externally and internally. 

 This was the result of the intercalation of numerous short vessels, 

 which arched across each node, their concavities being directed in- 

 wards, and which constituted the portion of the woody zone that 

 encroached upon the constricted pith at these nodes. Several modi- 

 fications of the above type have been met with, most of which may 

 have had a specific value. In one form no canals exist at the inner 

 angles of the woody wedges ; in another, laminae, like those of the 

 woody wedges, are developed in the more external portions of the 

 primary medullary rays, those occupying the centre of each ray 

 being the most external and latest formed. The primary ray is thus 

 transformed into a series of secondary ones. 



In another type the vascular laminae of each woody wedge are few 

 in nundjer, and the comy)onent vessels are the same ; but the latter 

 are remarkable for their large size. In a fourth variety, the ex- 

 terior of the woody zone has been almost smooth, instead of exhibit- 

 ing the usual ridges and furrows : this variety is also remarkal)le for 

 the large size of its metluUary cells, compared with that of the cells 

 and vessels of the wood}' zone. 



But the most curious modification is seen in a plant previously 

 described by the author under the name of Cahnnopitus, in which 

 round or oblong canals are given off from the medullar}^ cavity, and 

 pass horizontally through each primary medullary ray of the woody 

 zone to the bark. These, being arranged in regular verticils below 

 each node, are designated the infranodal canals. The verticils of 

 small round or oblong scars, seen at one extremity of the internodes 



