58 



ON THE PLANT-CELL. 



46 



with rings less distant ; then spiral vessels succeed, whose spires, al- 

 though far apart, are yet not so far apart as the rings of the annular 

 vessels ; then follow closely-wound spiral vessels ; then reticulated ; and 

 lastly, porous vessels. This course is observed although one or more of 

 these formations may be absent. So constant is this structure, that the 

 relative age of two vessels may be easily indicated by it. Thus in 

 monocotyledonous plants we often see porous vessels lying at the side, 

 or behind, the spiral and reticulated vessels, but they arise later than 

 the others, and this is shown by their configuration. 



a. In Monocotyledons, a remarkable change takes place in the cam- 

 bium within a year of the first period of vegetation. At the commence- 

 ment the cells which contained cytoblasts lose them, and their place is 

 taken by a clear fluid, and all new formations cease, and the cells become 

 arranged in perpendicular rows, so that where from three to five cells 



46 Simultaneous vascular bundles from the stem of Polypodium ramosum. A, Trans- 

 verse section. JB, Longitudinal section, through the smaller diameter of the vascular 

 bundle. The arrows show the direction from the centre to the periphery of the stem. 

 The thickened, somewhat lengthened, parenchyma which lies upon the vessels, surrounds 

 a thickened, very lengthened, parenchyma which represents the cambium in monoco- 

 tyledons. 



