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MISS NELLIE BANCEOFT ON 



Structure of the Steles. 



These are irregular in nature, in some respects recalling the plate-rings of certain 

 Medullosese. The stele 3 of the inner series, as seen in Section A, is a fairly typical 

 example (PL 10. fig. 2 ; PI. 11. fig. 3). At first sight, the impression is given of a 

 somewhat elliptical mass or bundle, apparently a single complete structure. On closer 

 examination, however, this is seen to consist of two closely-developed though distinct 

 parts, the inner being much larger than the outer. These are presumably the product 

 of two cambiums, one giving rise to normally, the other to inversely, orientated elements. 

 The fact that the outer and inner, or normal and inverse, parts are independent 

 structures is demonstrated by a slight space between them, and by the lateral dis- 

 continuity of the brownish elements, which are considered to represent crushed cambium 

 and phloem (PI. 10. fig. 2). The crushed tissue between the two masses shows here and 

 there traces of vascular elements, and may represent a much reduced " partial pith " 

 such as shown definitely in Medullosa stellata, Cotta, or M. jporosa, Cotta (13, Taf. vii. 

 figs. 1 & 4 ; p, 64, fig. 9), or it may represent ordinary ground-tissue with transfusion- 

 cells, such as occurs in Ci/cas, hetween the normal and inverse bundles of the stem. On 

 to this tissue abut what appear to be the protoxylems of the two masses (PI. 10. figs. 2 

 & 15). The inverse group shows a lateral projection, probably a leaf-trace about to 

 separate. The direction of the xylem elements is here slightly oblique (PL 10. fig. 2, oh; 

 PL 11. fig. 3). In the diagram the dark line marking the external borders of the xylem 

 represents the brown crushed cells, probably the remains of cambium and phloem. In 

 the case of stele 3 nothing could be determined with regard to the form and arrange- 



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ment of these cells. In Section B stele 3 shows very little change in form and structure ; 

 a slight interruption in the continuity of the radial series of elements appears in the 

 inverse part, which is larger in proportion to the whole stele than in Section A (PL 11. 



fig. 7, cf. fig. 8). 



A structure such as stele 3 may be compared with the plate-rings of Medullosa 



stellata, Cotta, which, it will be remembered, consist of a " partial pith " surrounded by 

 secondary vascular tissues. In stele 3 the partial pith may be regarded as either much 

 reduced in amount or crushed by subsequent secondary growth; while the lateral 

 portions of the complete ring may have remained undeveloped by discontinuity of the 

 cambium at these points. In this way a stele, consisting apparently of two distinct 

 parts, may be derived from a complete ring of the Medullosa stellata type. Worsdell (14) 

 considers that the so-called anomalous vascular structures in Cycas may be derived from 

 Medullosean steles by local suppression of cambiums*, and the type of structure 

 possessed by this South African stem may also have been produced in the same manner. 

 Weber and Sterzel's figures (13, Taf. viii. fig. 3; Taf. iii. fig. 5) of 31. Solmsii, Schenk, 

 indicate a similar condition in some of the steles of that form, partial pith being reduced 

 or practically absent, while there is also a smaller lateral development of the secondary 

 tissues of the rings. 



also Chodat ( 



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