1895.] Lyginodendron and Heterangiuin. 201 



the wood a zone of phloem can be traced, and this again is sur- 

 rounded by a parenchymatous belt, which we regard as pericycle. 

 The inner cortex is characterised by the presence of horizontal plates 

 of sclerotic tissue. The outer cortical zone has a structure similar 

 to that of Lyginodendron. In the pericycle and cortex numerous 

 leaf- trace bundles are met with. 



2. Course of the Vascular Bundles. The bundles can be traced 

 from the stele into the bases of the leaves. Their arrangement indi- 

 cates that the phyllotaxis was three-eighths in the larger and two- 

 fifths in the smaller stems. Each leaf received a single bundle. 

 The leaf-trace bundles can be followed downwards for some distance 

 at the periphery of the stele, where they form distinct strands, 

 though united with the axial wood. 



3. Primary Structure of the Stele and Leaf-trace Bundles. The 

 strands at the periphery of the stele, as well as the leaf-trace bundles 

 with which they are continuous, have the same collateral and meso- 

 xylic structure as the bundles in the stem of Lyginodendron, or the 

 foliar bundles of Cycadeee. The essential difference from Lyginoden- 

 dron consists in the fact that in Heterangium these bundles are 

 united by the axial xylem, which extends throughout the whole 

 interior of the stele. The primary tracheides, with the exception of 

 those adjoining the protoxylem, have numerous bordered pits. 



4. The Secondary Tissues. The secondary wood, when present, 

 has essentially the same structure as in Lyginodendron. Cambium 

 and phloem, in the normal position, are fairly preserved in some of 

 the specimens. 



5. The Cortex. The most characteristic feature here consists in 

 the horizontal plates of sclerotic cells in the inner cortex. Their 

 structure is precisely that of the " stone-cells," found in the cortical 

 tissues of many recent plants. Their presence in the cortex of the 

 stem ia a point of difference from Lyginodendron, where the sclerotic 

 masses are usually limited to the pith and pericycle. 



6. Branching of the Stem. In one specimen a young stem bears a 

 branch much smaller than itself. This is the only distinct case of 

 branching observed in either genus. All other supposed branches 

 have turned out to be either petioles or adventitious roots. 



B. The Leaf . 



1. Connexion between Leaf and Stem. The bases of petioles, in 

 connexion with the stem, have been observed both in transverse and 

 longitudinal section. These specimens show that the cortical tissues 

 of the petiole have the same characteristic structure as those of the 

 stem ; we are thus enabled to recognise the petioles of Heterangium 

 Orievii when detached from the stem. Unlike Lyginodendron, the 



