430 SCIENCE PROGRESS. 



bays of the primary wood, soon extends to the fascicular 

 regions, and thus completely encloses the axial strand. 

 The amount of secondary wood naturally varies con- 

 siderably in different sections, the tracheids in a single 

 radial row varying from one to thirty-seven in number. 

 The medullary rays either extend as continuous lines of 

 parenchyma through the whole thickness of the wood, or 

 occur in the form of cell groups at the angles of the tracheids ; 

 in the latter case the apparently isolated clusters of paren- 

 chyma are united by connecting cells stretching across the 

 radial walls of the reticulately pitted tracheids. Owing to 

 the smaller diameter of the fascicular tracheae, the secondary 

 xylem exhibits a fairly obvious division into six groups, 

 three broader masses of interfascicular tracheids, alter- 

 nating with three smaller groups of radial rows of fascicular 

 tracheids, tapering towards the protoxylem angles of the 

 primary xylem. 



The formation of periderm is another characteristic 

 feature in the secondary growth of a Sphenophyllum stem. 

 A phellogen or cork cambium appears to arise in the 

 pericycle, and at a later stage the phloem parenchyma 

 takes part in the development of cork tissue. It is often 

 a matter of some difficulty to distinguish between the true 

 phloem and the internal periderm. The latter consists of 

 short cells in regular series, the former being made up of 

 much longer elements, which may possibly be sieve-tubes. 



Leaves. — The most perfect example of a petrified leaf of 

 Sphenophyllum so far described, is one figured by Renault. 1 

 In transverse section the lamina is seen to be composed of 

 thin-walled loose parenchyma, with small groups of trac- 

 heids marking the position of the veins. The epidermal 

 layer on the upper and under surface consists of fairly 

 thick-walled cells, with indications of stomata. The most 

 distinctly preserved stoma has, however, been figured by 

 Solms-Laubach 2 from the epidermis of one of the leaf 

 segments of a strobilus ; in this there are two narrow guard 

 cells with two larger subsidiary cells. 



1 Renault (i), pi. ix., fig. 6; and (2), pi. xvi., fig. 1. 



2 Solms-Laubach, pi. x., fig. 9. 



