328 VEGETATIVE ORGANS OF LYGINOPTERIS 



separate fragments of stem, root, leaves, etc., are studied as 

 unrelated structures, but subsequently patient toil pieces them 

 together till a more or less complete picture of the whole is 

 obtained. The external appearance of a fossil plant is best 

 appreciated from casts or impressions preserved in consolidated 

 mud, sand, etc. Not infrequently, however, the tissues have 

 been impregnated with silica, calcium carbonate, etc., so that 

 the internal structure is recognisable. Such petrified portions 

 are cut up into thin slices, and these are ground down until 

 sections, comparable to those prepared from a living plant, are 

 obtained. 



' Lyginopteris (Lyginodendron) oldhamia, one of the commonest 

 of the Coal Measure fossils, has been pieced together in this way 

 until it is known with a degree of completeness only shared by a 

 few living plants. Lyginopteris was probably a woody scrambler, 

 with relatively slender stems and large compound Fern-like 

 leaves (Fig. 188), the whole surface being beset with spines and 

 large glandular hairs. The leaves were separated by long inter- 

 nodes and produced occasional axillary branches, whilst the 

 stem was attached to the soil by a number of adventitious roots 

 arising near its base. The microsporangia and megasporangia 

 (ovules) were borne on the ultimate ramifications of the ordinary 

 foliage-leaves (cf. Fig. 190). 



The stem underwent considerable secondary thickening. In 

 transverse sections (Fig. 189) the parenchymatous pith, which 

 included patches of sclerotic tissue, is seen to have been sur- 

 rounded by five or more groups of primary wood (X.), some of 

 which occurred in pairs ; in each such strand of primary xylem 

 the protoxylem occupied a more or less central position. Beyond 

 was a prominent zone of secondary wood (Sec.), with wide paren- 

 chymatous rays, through which the leaf-trace bundles emerged 

 (Bs.). The delicate cells of the cambium and phloem, external 

 to the wood, are rarely well preserved. A layer of cork-like 

 tissue (periderm) was usually developed in the pericyclic region, 

 whilst the thin-walled inner cortex contained numerous secretory 

 cells. A sharp contrast is afforded by the outer cortex, with 

 its system of radial sclerenchymatous plates (S.), anasto- 

 mosing at frequent intervals and giving mechanical support to 

 the stem. 



