78 



FOSSIL PLANTS. 



ago, some of this structure was noticed in tlie medulla of that plant, intermixed with the 

 scalariform tissue forming the bulk of the central axis ; and a somewhat similar structure 

 is mentioned as composing the medulla of Calamodendron commune, in Specimens Nos. 3 

 and 4, at page 23 of Part I of this Monograph. I have also observed it in the medulla 

 of a small stem which would pass for a 8tigmana ; and it has been there sometimes 

 mistaken for common parenchymatous tissue. For the sake of convenience, and in order 

 to distinguish it from ordinary parenchyma and prosenchyma, I purpose to term it 

 " orthosenchyma," and " orthosenchymatous tissue," in the following descriptions.^ 



The medulla of orthosenchymatous tissue is surrounded by a zone of scalariform vas- 

 cular tissue, of large dimensions in the interior, and gradually diminishing in size as it 

 approaches and enters the dark corrugated line on the exterior. From this part originate 

 the vascular bundles which communicate with the leaves. They appear to come from the 

 concave spaces forming the outer edge of the dark line. Succeeding the vascular cjdinder, 

 which is quite destitute of any traces of medullary rays, is a dark shaded zone, of about 

 twice the diameter of the internal woody cylinder previously described, composed chiefly 

 of the mouths of the vascular bundles, but also exhibiting a series of round masses of 

 large cells, of a light colour, which are only slightly shown in the plate (fig. 1). Next 

 comes the great mass of fine parenchymatous tissue, traversed by vascular bundles ; but 

 this in the Hesley Heath and most other specimens has for the most part disappeared, 

 having been replaced by mineral matter. This zone of tissue increases in size and 

 strength as it approaches the broken and irregular line seen near to the exterior of the 

 specimen. Outside this line the tissue gradually enlarges into long utricles, or tubes, 

 arranged in a radiating series. This is covered by a thick epidermis, now converted into 

 bright coal, showing no traces of structure. 



Fig. 2 is a longitudinal section of a portion of the stem (magnified four diameters), 

 extending from the inside of the woody cylinder (b), through the zone of fine vas- 

 cular tissue (c), whence arise the vascular bundles {d), which traverse the mass of paren- 

 chymatous tissue (e) out to the leaves. These bundles at first run nearly parallel to the 

 woody cylinder, then gradually curve outwards, until they become nearly level, when they 

 again curve upwards, and reach the outside of the stem, and communicate with the 

 leaves. 



Fig. 3 is another longitudinal section of the inner portion of the stem (magnified seven 

 diameters), showing the orthosenchymatous tissue (a) of a part of the medulla, the zones 

 of coarse and fine vascular tissue (5 and c), and the vascular bundles of scalariform tissue 

 {d), surrounded by a zone of fine orthosenchymatous tissue similar in structure (but finer 



^ Probably some may object to the coining of a new term, and think that orthosenchyma is merely 

 muriform tissue set upon its end ; but that is scarcely so ; for the short cross lines are not placed at right 

 angles to the long ones, as is generally the case in the latter structure, but at diiferent angles ; and it cannot 

 well be termed cubical cellular tissue. 



