TRANSACTIONS OF SECTION K. 869 



5. The Morphology of the Central Cylinder in Vascular Plants, 



By E. C. Jeffrey. 



There are three main types of fibro-vascular arrangement in plant axes, viz. — 

 (I) a single so-called concentric aggregation ; (2) several such aggregations, com- 

 monly, but not always, grouped in a circle ; (3) a ring of so-called collateral or 

 bicoUateral bundles. 



Disregarding the older views, Van Tieghem's is that the first type is primitive, 

 and is to be designated monostelic; the second is derived from it by simple 

 multiplication, and is consequently to be considered as polystelic ; the last type 

 is derived from the first by the segregation of a parenchymatous pith in the 

 midst of the central vascular core, and the splitting of the resultant fibro- 

 vascular ring into wedges called fibro-vascular bundles by radiating parenchy- 

 matous strands, the medullary rays. The last type of stem is according to this 

 conception monostelic. 



The writer considers that this view of the phenomena does not correspond 

 with morphological facts. In some of the Pteridophyta we have a single vascular 

 non-meduUated axis from which originate the leaf-traces. This state of affairs is 

 found, for example, commonly in the genera Lycopodium and Selaginella. In many 

 ferns the stele becomes a tube just below the origin of the leaf-trace, and this 

 cylinder breaks open again above the exit of the fohar bundle. This modification 

 is seen in its simplest form in ferns with sparse leaves and creeping rhizomes, and 

 the tubulisation of the stele seems to be a mechanical device to strengthen the 

 slender axis, and to enable it to support its comparatively enormous leaves. 

 Where the leaves are close ranked and the stem ascends the foliar gaps overlap, 

 and the stelar cylinder becomes in a transverse section apparently a circle of 

 separate bundles. In all such cases examined by Le Clerc du Sablon and the 

 writer, the young stem has a tubular stele. Even when widely separated the 

 vascular strands anastomose, so that there results a mechanically efficient fibro- 

 vascular cylinder. In some genera, for example, Antrophyum and Vittaria, the 

 internal bast of the stelar tube degenerates, the result being a state of affairs 

 approximating that found in the Angiosperms with the exception that the pith 

 freely communicates with the outside. This modification is very marked in 

 certain Ophioglossacefe and Lepidodendraceae, which will be described at length in 

 the fuller account which will appear shortly. In the former there is present 

 sometimes an internal endodermis, although the internal bast has disappeared. 

 Among the Gleicheniacese we have in Mertensia the cortex sending parenchy- 

 matous strands into the vascular axis of the stem down through the channelled 

 leaf-traces; in Gldchenia and Platyzoma these are completely cut off from the 

 outside cortex, and we find an entirely included pith similar to that presented by 

 Osmunda, The pith of these forms is in reality extrastelar, but no longer com- 

 municates with the peripheral cortex. 



We have a similar series in the Equisetaceae, where in the young stem the 

 vascular axis is not primitively dialjdesmic, but gamodesmic, contrary to the 

 statement of Van Tieghem. The primitive stelar arrangement is a closed tube 

 with external and internal endodermic, but no internal bast. In the older stem 

 this condition may be replaced by isolated bundles, surrounded by individual 

 endodermal sheaths. Often, however, the primitive tube remains intact, and the 

 internal endodermis disappears, bringing about a disposition quite similar to that 

 obtaining in the higher plants. It \s interesting to note that the Calamitece pre- 

 sented a still closer resemblance to the latter, owing to the presence of secondary 

 wood. 



The writer proposes for the stelar tube the term siphonostelic ; where the 

 internal as well as external bast is present, the stele is to be described as amphi- 

 phloic ; where only the external bast is present, ectophloic. Monostelic axes are 

 to be considered as protostelic, and present a marked contrast to the mechanicallj^ 

 modified siphonostelic axes. 



In the Filicales the siphonostelic modification arose in connection with the 



