Palaeontolosie. 215 



'& 



from one form to the other has been observed. Secondly that there 

 are two distinct though closel}'' allied species; but the points ol' 

 resemblance are too numerous to justify such a conclusion. Thirdly 

 that the types are habitat forms of a Single species and throw light 

 on the ecology of R. cylindrica. The reduction of the xylem 

 Strand and its concentration, as exemplified by the presence of only 

 one protoxylem group; the relatively wide cortex; the production 

 of airspaces; and the absence of mechanical tissue features which 

 characterize both stems and petioles of the f? type, as compared 

 with those of the « type — are modifications which find their pa- 

 rallel among waterdwelling forms of recent plants. It is therefore 

 suggested that R. cylindrica was amphibious, « and ß plants being 

 respectively its land and water ecads 



R. cylindrica seems to be closely related to B antiqiia, B. ra- 

 rnosa, and B. hirsuta. So far as the foliar trace is concerned, the 

 four species form a progressive series from the relatively primitive 

 B. ayitiqua to the tridentate types, R. cylindrica representing an 

 intermediate term. B. forensis does not appear to be ver}?^ closely 

 related to this group of British species. 



Concerning the nature of the primitive stele it is impossible to say 

 with certainty whether the stele of R. cylindrica is more or less 

 highly organized than those of related species. It can, however, be 

 stated that typical steles of R. cyWidrica show some divergence 

 from the primitive condition , whefher this is considered to be an 

 endarch or an exarch protostele, or an asterostele. 



A comparison of the methods of stem-branching and leaf- 

 production in R. cylindrica provides evidence in favour of the vievv, 

 suggested by Bower in 1884-. that stem and leaf are homologous 

 branches of a primitively undifferentiated and dichotomous System. 

 Stem-branching and leaf production have been described in R. cy- 

 lindrica, and it is evident that the two processes are essentially the 

 same in origin. In branching, however, the completion of both 

 branch steles is ensured by the formation of metaxylem elements 

 below the actual level of their Separation; in leafproduclion, on the 

 other band, only the stera siele is completed in this way, for at 

 the place of Separation the leaf-trace does not appear to possess 

 any adaxial metaxylem. A few elements may be developed higher in 

 the petiolar trace, but at a slightly higher level still they tend to 

 disappear again; their formation may be regarded as indicating an 

 earlier condition similar to that seen in B. antiqua, in which some 

 adaxial metaxylem is present at the level where the trace separates 

 from the stem stele, and which is therefore still more suggestive 

 of modified stem-branching. 



This view of the origin of the leaf is further supported by the 

 similar behaviour of the protoxylem in branching and in leaf for- 

 mation. In «« types, the protoxylem group of both branch- and leaf- 

 traces divides more or less definitely; in 6 types, no division nor- 

 mally takes place in either instance, W. B. Turrill (Kew). 



Holden, R., Jurassi c wood from Scotland. (New Phytologist. 

 XIV. 6 and 7. p. 205—209. June and July 1915) 



A portion of a stem, at least 75 years old, from the Corallian 

 of Loth was sectionised and the anatomy described. The annual 

 rings are well marked and the spring wood grades evenly into that 

 formed in summer, indicating that the organ is a stem not a root. 



