842 MR JOHN M'LEAN THOMPSON ON 



The irregularity in the number of leal-trace bundles has been referred to, and it 

 should be noted that although five bundles are frequently found to constitute a trace 

 on its way out through the cortex, this number is usually established by the division 

 into two of one of the four bundles arising directly from a leaf-gap. In like manner 

 a leaf-trace originally consisting of three bundles is usually possessed of four or five 

 strands before it has passed into its leaf-base. 



The two large bundles which take up an adaxial position in the leaf-base and 

 rachis arise constantly one on each side of the leaf-gap, seldom close to its summit, 

 and frequently at different levels. For the remaining bundle or bundles, whose 

 position in the leaf-base and rachis is abaxial, the insertion is either at the base of 

 the gap or on its side or sides. The leaf-gaps represented in the reconstruction will 

 give some idea of the variety of detail seen in the gap-form and in the leaf-trace, 

 The long leaf-gap figured on the right of the reconstruction is particularly worthy 

 of note, bearing, as it does, roots on its margins towards the top, and a leaf-trace of 

 four bundles towards its base. 



The perforations are as variable in form and size as are the leaf-gaps, and are 

 irregularly distributed (fig. 40 and text-figs, i-xxvi). They are always smaller than 

 the leaf-gaps, and range from minute circular piercings of the stele to fairly large, 

 elongated, irregular openings. 



There is some variety in the histological details of the meristeles. In those parts 

 of the vascular system showing advanced dictyostely each meristele usually appears 

 in transverse section as a solid central mass of tracheides surrounded by a narrow 

 zone of small-celled phloem, which is, in turn, enclosed in a double row of large- 

 celled pericycle delimited from the cortex by an endodermis of brown, barrel-shaped 

 cells. But at times the small meristeles, which appear circular in transverse section, 

 have a core of parenchyma through which a few tracheides are scattered, while the 

 main body of xylem forms a closed ring around the parenchyma. In those parts in 

 which a less advanced dictyostely is found, and the meristeles are narrow and 

 elongated in transverse section, the parenchymatous replacement of tracheides is 

 even more extensively developed, and the pericycle is less conspicuous and smaller 

 celled than in the much divided portions of the stele. 



It will be evident that the vascular system is of an advanced type, and is 

 characterised by pronounced plasticity of form. 



The Root. 



The roots are uninteresting. They are thin, black, and fibrous, and seldom 

 branch. The cortex is about twelve cells broad, and its inner zone is a small-celled 

 sclerotic band. The endodermal cells are narrow and tangentially elongated, while 

 the pericycle usually consists of two layers of large, thin-walled cells. The xylem is 

 diarch, and appears in transverse section as a solid mass of tracheides with broad 



