322 FOSSIL PLANTS OF THE COAL-MEASURES. 



admirable drawings in plate xvi. fig. 4. The nature of the secondary 

 tissues has been satisfactorily determined from an examination of 

 numerous roots in different stages of development. On the whole, 

 the secondary wood is identical with that of the Calamite stem ; 

 immediately opposite to each protoxylem group there is usually 

 found a medullary ray, which in some cases may be traced for a 

 long distance through the wood. The presence of this medullary 

 ray affords another piece of confirmatory evidence as to the practical 

 identity in structure between the roots of Calamites and those of 

 recent plants. Of the phloem, fewer and less distinct traces have 

 been detected. Passing to the cortical tissues, a definite structural 

 feature is recognised in the three well-marked zones ; an inner 

 cortex of continuous parenchyma, a middle lacunar cortex, and 

 an outer zone of continuous parenchymatous tissue. 



In some of the smaller root-branches, an extremely interesting 

 feature is pointed out hi the innermost layers of the cortex ; next to 

 the primary phloem there occur two layers of cells corresponding 

 in position to the endodermis and pericycle, but the cells of the two 

 layers "fit on to each other as if they had had a common origin." 

 This suggests the double endodermis characteristic of Kquisctum, 

 and, if established, affords another illustration of the very intimate 

 connection between the arborescent equisetaceous plants of the 

 Coal-Measures and the recent Horsetails. 



In the so-called rootlets of Caiainites the pith of the older roots 

 is usually absent ; and the structure may be diarch or tetrach. In 

 the thicker roots as many as twenty-five groups of primary xylem 

 and phloem occur. The outermost cortical layer is characterised 

 by thick external walls, and is spoken of as the epidermoidal layer. 

 It would seem that a layer of more delicate cells was originally ex- 

 ternal to this protective epidermoidal tissue, but was cast off' at a 

 comparatively early stage of development. 



The genus Calamites affords one of the most striking examples 

 among paUeozoic plants, of the very great importance of the application 

 of detailed histological methods in palseobotanical investigations. 



The memoir on Lyt/inoilcndron and HetevaiKiinm is undoubtedly 

 one of the most important contributions to fossil botany published 

 in recent years. The interest attached to the two genera is one 

 which every student of plant evolution cannot fail to appreciate. 

 None of the records of the past are of such importance as those 

 which enable us to bridge over gaps between existing classes of 

 organisms. We have previously learnt something of intermediate 

 plant types, <;.//. the palaeozoic genera Poroxyhm, Protupiti/s, Mije- 

 Inxyhm, and others ; but of Li/f/iiiodendroii and Heterannium our 

 knowledge has been rendered much more complete, and many new 

 characters of primary importance are clearly demonstrated in this 

 admirable piece of investigation. 



Lrjginodcndron. — Fragments of stems, petioles, and foliage of 

 this genus are fairly abundant in the calcareous nodules or coal-balls 

 of Lancashire and Yorkshire. Although no undoubted examples of 

 impressions or casts have so far been recognised, it is possible to 

 restore with a considerable degree of probability the general form 



