1895.] Lyginodendron and Heterangium. 197 



their structure became concmtric, the phloem here extending all 

 round the xylem. 



The xylem of the bundles in the stem of Lyginodendron exactly 

 resembles that in the leaves of existing Cycadeae. The protoxylem 

 lies in the interior of the primary wood, but near its outer side, so 

 that the greater part of the primary wood was centripetally developed, 

 while a smaller portion was centrifugal. We propose to term such 

 bundles mesoxylic or mesarch.* All statements as to the position of 

 the protoxylem are based on longitudinal as well as on transverse 

 sections. 



4. The Secondary Tissues. A few young stems have been observed 

 with little or no secondary thickening ; in most specimens it has 

 made considerable progress. A large amount of secondary wood and 

 bast, both fascicular and interfascicular, was formed, by means of a 

 normal cambial layer, which is often well preserved. 



The tracheides of the wood have numerous bordered pits on their 

 radial surfaces. Similar elements occur in the primary wood also. 

 The rows of tracheides are separated by numerous medullary rays. 



The phloem is often well preserved, so that primary and secondary 

 phloem can be distinguished. 



The secondary tissues bear a general resemblance to those in the 

 stems of Cycadese. 



5. Pith and Pericycle. Both these tissues contained nests of dark 

 coloured elements, probably of a sclerotic nature. They are also 

 traversed by numerous rows of cells with carbonaceous contents, 

 which may have been secretory sacs, but not intercellular canals. At 

 the outer border of the pericycle a characteristic internal periderm was 

 developed. 



6. The Cortex. The parenchymatous portions of the outer cortex 

 became much dilated in the older stems, in consequence of the 

 secondary growth. 



7. On Small Stems of the Lyginodendron Type. Certain very small 

 stems have been described, differing in structure from the usual 

 form. In some of these the primary xylem forms a continuous ring, 

 instead of being divided into distinct bundles. We now suggest that 

 these specimens may represent the basal, first-developed region, of 

 normal stems. In Osmunda, which in many respects resembles 

 Lyginodendron, it has been shown by M. Leclerc du Sablon, that the 

 embryonic stem has the same peculiarity. 



8. Structural Anomalies. Some of the specimens show remarkable 

 individual anomalies, the most frequent and conspicuous of which 

 consists in the appearance of a cambium at the periphery of the pith, 



* One of the authors has recently found that this peculiarity sometimes extends 

 to stem-structures in Cycadese ; in the peduncles of both male and female flowers of 

 Stangeria the bundles are often mesoiylic. 



