THE VASCULAR TISSUES. 



77 



areas separated by bars of thickened wall ; these thickenings become, 

 further down, the annular and spiral bands of the elements of the 

 protoxylem (see Fig. 60). External to these fusiform cells are to be seen 

 somewhat elongated cells filled with protoplasm and with long spindle- 

 shaped nuclei. These cells form the rudimentary meristem and phloem 

 elements, which, further down the shoot, are differentiated into young 

 cambium and protophloem. 



In this connection it is interesting to note that spiral ^essels occur ia 

 such plants as the Mosses and Liverworts. In the simple leaf of Funaria 



Fig. 61. — Portion of a Moss-leaf, showing (in optical section) indica- 

 tions of a simple vascular system. — xy, Spiral vessel ; p, protective 

 cells just outside the vessels ; c, cells of the green assimilating tissue 

 of the leaf (mesophyll). 



a few elements having the characteristics of the spiral vessels, with rather 

 broad thickening spirals, are to be seen towards the central axis of the 

 leaf (see Fig. 61). These are surrounded by a few elements of a thick- 

 walled nature, and outside these latter comes the green assimilating tissue 

 of the leaf. Nevertheless the Bryophyta are not included amongst th& 

 vascular Cryptogams. 



