220 HOF5IE1STER, ON 



of the thickenings of the wall is in the form of delicate 

 transverse streaks, and commences long before the termina- 

 tion of the longitudinal growth of the cell, and even during 

 the existence of the parietal nucleus and of the strings of 

 granular mucilage proceeding from it (PI. XXXI, fig. 1). 



Long before the appearance of the first traces of the 

 thickenings of the walls of the scalariform vessels, spiral 

 thickenings are visible in certain cells arranged in groups 

 of twos or threes and which at an early period become 

 spindle-shaped. It is very clearly seen that the formation 

 of the spiral band proceeds gradually from the lower to the 

 upper end of the cell (PI. XXX, fig. 12). In each vascular 

 bundle these small groups of spiral vessels are formed : one 

 axile group is formed in those vessels which are circular in 

 a transverse section, and three are usually formed in those 

 bundles whose transverse section is elongated ; one of such 

 groups being in the centre, and the others in the foci of 

 the figure presented by the transverse section, and which 

 bears a distant resemblance to an ellipse. 



The great expansion of the cells of the vascular bundle 

 which go to form the scalariform vessels, causes such a com- 

 pression of the intermediate, narrow, prosenchymatal cells, 

 that in some instances the entire cavity of the latter is 

 obliterated. 



A transverse section of the youngest portion of a vascular 

 bundle taken at a point near the terminal bud where the 

 thickening layers are visible only in the spiral vessels, 

 exhibits a considerably larger number of cells than is seen 

 in the same vascular bundle at a distance of about a line 

 and a half from the terminal bud after its scalariform vessels 

 are completed (PL XXX, figs. 10, 10' ; ). A similar state of 

 things exists in the vascular bundles of the stipes. Trans- 

 verse sections of the compressed cells, provided the cavities 

 of the latter are not quite obliterated, bear a considerable 

 resemblance to the lenticular cavities between two pits of 

 coniferous wood (PI. XXX, fig. 2, between the two wide 

 vessels) . 



The course of the vascular bundle nearest to the middle 

 of the stem (i. e. of the upper one of the two primary ones), 

 is almost exactly parallel to the longitudinal axis close 



