488 



On the Structure of Roots. 



Fig. 15. 



plication by division ; this is the chief focus of development of 

 the root. Continuing the examination upward to the older part 

 of the root, the rudimentary cells are soon found arranged in 

 rows parallel to the direction of the root ; at first they are very 

 short, then squarish in the side view, and by degrees they are 

 elongated until their length is much greater than their breadth ; 

 they also expand laterally to a certain extent after tlieir first 

 formation ; but this growth ceases, so that the rootlet has a 

 fixed diameter. Tiie cell-division seems to be repeated in these 

 cells in the direction of their length after they have attained their 

 full diameter. While young, near the tip of the root, they are 

 densely filled with protoplasmic substances; as they expand they 



appear clearer, and 

 contain only a moder- 

 ate quantity of proto- 

 plasm, witli abundant 

 watery cell-sap. The 

 rudimentary cells de- 

 veloped in the very 

 centre of the point of 

 growth become cells 

 of much less diameter 

 and more elongated 

 form, and constitute 

 the rudiment of a 

 fibro - vascular cord 

 running through the 

 centre of the rootlet ; 

 at a little distance 

 from the point, traces 

 of spiral markings 

 may be detected on 

 the walls of some of 

 these cells, which are 

 becoming vessels, the 

 distinguishing marks 

 of the fibro-vascular 

 bundles. Higher up 

 in the root, the central 

 fibro-vascular cord is 

 clearly recognizable, 



a, cross, and 6 perpendicular, slices of a root of the onion, mag- SUrrounoed by paren- 

 nified 50 diameters. In the centre is the fibro-vascular axis, chymatOUS Cells, them- 



with the ducts on the outside. g^l^^g enclosed by a 



continuous layer of delicate epidermal cells (fig. 15). In these 

 roots the epidermal cells do not grow out in hairs (radical fibrils). 



