186 TKAXSAUTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



Notes on Leaf-sections of Polytrichum. 



By John 11. Lee. 



[Read 23rd Juue, 1908.] 



TuE Polytrichacete represent the highest type of organisation 

 amongst Mosses ; and, in fact, present us with the most highly 

 organised form of the gametophyte or sexual generation in 

 plants. Not only is there a robustness of aspect in most of the 

 species, which is almost unique, but the histology of the stems 

 and leaves presents a complexity of structure far surpassing that 

 of any other group of the Bryophyta. Although it cannot be 

 said that there is anything quite comparable with the elaborate 

 tibro-vaseular tissues of the plant-body in Phanerogams and 

 Pteridophytes, yet there are present in the conductive system of 

 the Polytrichacese elements which recall the fibres, vessels, and 

 and sieve-tubes of the higher plants, and have, in fact, been 

 regarded as analogous structures by some observers. 



Connected with this highly-specialised conductive system, it is 

 not surprising to find a type of leaf-structure more elaborate than 

 is usual amongst the other groups of mosses. Although, in 

 general, the central mib-rib or " nerve " of the leaf presents a 

 certain amount of complexity, yet in the great majority of mosses 

 the leaf is extremely simple in structure, the greater part usually 

 consisting of a simple plate of cells, filled with chloroplastids. In 

 the Polytrichaceffi, on the other hand, the "nerve," which is 

 unusually large, beai'.s, on it ventral surface, a number of plates, 

 or "lamellae," running parallel to each other in a longitudinal 

 direction, and appearing in transverse section as a series of 

 upright I'ows of cells. In the genus Catharinea, these lamellae 

 are few in number — not more than four or six — and are compara- 

 tively narrow, the broad lamina of the leaf considerably exceeding 

 the lamellffi in extent of surface ; while the cells of the lamellse, 

 as well as those of the leaf-blade itself, contain abundant chloro- 

 phyll. In the genus Folytrichmn, on the other hand, the lamellae 

 are very numerous — as many as sixty in P. commune — and are 

 the only chlorophyllose parts of the leaf, the lamina being in 

 many cases reduced to a very narrow wing at the extreme edge 

 of the extended '■ nerve." 



