364 ME. ABTHUE JOHN MASLEN ON THE 



The tracheas in each bundle vary in number between about 10 and 16, and 

 the smaller elements certainly in many cases appear to occupy an internal position, so 

 that we have here, probably, another example of the mesarch type of vascular bundle 

 (see PI. XXXVI. fig. 12) commonly found in the foliar bundles of recent Lycopods * 

 and Cycads, as well as in many of the old Palseozoic Cryptogams f . 



When passing through the inner space (PI. XXXVI. fig. 2, i.s.) the bundles, l.t., show an 

 investment of fine parenchymatous cells extending completely round thera. On 

 PL XXXVI. fig. 5, at l.f'., is shown a similar bundle in the inner space, and here, too, it 

 is seen to have a parenchymatous investment of cells even where quite free from the 

 central parenchyma. The bundle shown on PL XXXVI. fig. 2, a, at the inner margin of 

 the inner cortex, still retains an almost complete investment of these thin-walled cells. 

 So there seems no doubt that the outgoing leaf-trace bundles are accompanied by 

 an investment of parenchymatous cells, and that these cells are continuous with those 

 immediately surrounding the xylem cylinder of the axis of the cone. 



Leaving the inner space, the bundle next traverses the inner cortex. The transverse 

 section (PL XXXVI. fig. 2) shows how the bundle, enveloped in its investment of cells, 

 gradually passes outward towards the inner cortex, which gradually bulges out into a 

 bay into which the bundle enters. It is here that we first have evidence of the collateral 

 structure of the bundle, the space on the outer side of the xylem and between it and the 

 inner cortex doubtless representing the position formerly occupied by the phloem. As the 

 bundle passes through it, the cells of the inner cortex gradually close up behind and the 

 bundle carries away with it a complete sheath of cells. This is also shown in longitudinal 

 section on PL XXXVI. fig. 5, where, at l.t"'., a leaf-trace bundle is shown in connection 

 with the inner cortex, i.e. 



It appears, then, that each bundle is provided with two sheaths — one surrounding the 

 xylem only and continuous with the tissue immediately investing the central xylem 

 cylinder ; the other, surrounding both xylem and phloem, consisting of somewhat thicker- 

 walled and larger cells continuous with the inner cortex. In the single bundle from 

 the middle cortical zone shown on PL XXXVI. fig. 12, these two sheaths are shown 

 at p. and i.c.sh. 



In the middle cortex-space the bundles have a diameter (including the inner cortex- 

 sheath) of about -15 millim. (C.N. 1613, 1613 r^, S. 85), and the sections show (see 

 PL XXXVI. fig. 12) that the cells of the inner cortical sheath, i.c.sh., are larger at the 

 inner side of the bundle, and that they gradually become smaller towards the outer side of 

 the trace. PL XXXVI. fig. 11 is a longitudinal section through a bundle in this region. 

 The larger cells of the inner cortical sheath are shown at the inner side of the bundle at 

 Lc.sh.,and the smaller somewhat more elongated ones at the outer side at i.c.sh/; x. is the 

 xylem, exhibiting the usual scalariform thickening, and /. the phloem, here preserved. 

 The latter consists of exceedingly thin-walled elements, elongated in the vertical direction 



* I am indebted to the kindness of Mr. L. A. Boodle, A.R.C.S., for having shown me (in the JodreU Laboratory, 

 Kew) Bome sections of the leaf of Lycoj_)odlmn in which the bundles show this structure very clearly. 



t See "Williamson and Scott, " Further Observations on the Organization of the Fossil Plants of the Coal Measures, 

 Part III., Lygimdendron and Eeterangium," Phil, Trans. B. 1896, p. 713. 



