BORDERED PITS 311 



teristic of narrow slit-like pits. The pits are sometimes placed at wide 

 intervals, while in other cases they are crowded closely together ; in 

 the latter event the border often acquires a polygonal outline. 



Hitherto discussion has been confined to the typical " two-sided " 

 bordered pits, such as occur on a wall which separates two vessels or 

 tracheides from one another, Wherever a tracheal that is, a water- 

 conducting element abuts against a parenchymatous cell engaged in 

 the storage or conduction of plastic materials, there one meets with 

 " one-sided " bordered pits, that is to say, pits which are only bordered 



Fig. 129. 



Water-conducting tissues of mosses. A. Central strand of the seta of Meesea 

 longiseta, consisting of a bundle of tracheides enclosed in a protective sheath. B. 

 Central strand of Rhynchostegium murale. C. A ribbon-shaped leaf-trace-bundle from 

 the cortex of the stem of Milium rostratum. (All in T.S.) 



on the side of the wall that pertains to the tracheal element. In such 

 cases the limiting membrane is never provided with a torus, but 

 remains thin all over ; the turgor-pressure in the parenchymatous 

 cell causes it to bulge towards the border. Under certain conditions 

 the limiting membrane may even grow out into a tubular process, 

 which penetrates right through the pit-cavity and protrudes into the 

 lumen of the vessel ; this phenomenon is considered in more detail 

 below (II. A. 5). 



The present section may conclude with a brief account of the 

 rudimentary water-conducting tissues which are frequently found in the 

 stems and setae of Mosses. 100 The author has proved that the cells 

 composing the simple central strand which is characteristic of the 

 axial organs of Mosses (Fig. 129), are to be regarded as rudimentary 

 tracheides. The cells in question are elongated prosenchymatous 

 elements, with narrow cavities, and usually with thin and delicate walls. 

 The oblique transverse walls in particular are always thin, even where 



