HETERANGIUM 409 



in Lyginodendron, so that a considerable number — 

 usually about eight — are met with in any transverse 

 section of the stem (Figs. 154 and 155, l.t). They are 

 of large size, and closely resemble the leaf-traces of 

 Lyginodendron, except that they are usually without 

 any secondary tissues. On leaving the stele the leaf- 

 traces are collateral, but become concentric before 

 entering the base of the leaf itself. They soon pass 

 out from the pericycle into the cortex, atid are most 

 frequently found in the latter tissue, whereas in Lygino- 

 dendron they follow the pericycle for a longer distance. 

 The trace usually remains undivided throughout its 

 course ; this, however, is only a specific character, for 

 other forms of Heterangium resemble Lyginodendron in 

 having twin leaf-traces. 



2. The Leaves. — The leaf-bases are constantly found 

 in connection with the stem, to which they give an 

 angular form, as they are decurrent, and the sharp 

 projection which marks the position of each leaf in the 

 stem extends a long way down — probably to the next 

 leaf of the same vertical series. Hence, when the 

 phyllotaxis is f, the stem has eight more or less 

 prominent angles, corresponding to the orthostichies 

 (Figs. 154 and 159). 



The structure of the petioles is well shown in those 

 which are still attached to the stem, and the knowledge 

 thus gained enables us to identify the free specimens 

 with certainty. The petiole is traversed by a single 

 vascular bundle of concentric structure. The inner 

 cortex contains conspicuous horizontal plates of scler- 

 otic tissue, like those of the stem, while the hypoderma 



