THE FIBRO-VASCVLAR BUNDLES OF ROOTS. 



^35 



tissue, to which I shall come back later. It would carry us here much too far to 

 enter more in detail into the relations indicated : moreover this would be superfluous, 

 since their physiological significance, as already explained, are either not known, or, 

 where it is possible, will be brought forward subsequently as opportunity serves. 



Strikingly different from the vascular bundles of the shoot-axes and leaves are 

 the fibro-vascular strands of all true roots; and they are usually so characteristic 

 that it is recognised at once on a transverse section whether a root-strand is under 

 observation or not. The most striking feature is that the xylem and phloem-bundles 

 are so placed in groups at the periphery of the axial cylinder, that they alternate 

 with one another laterally ; the phloem-bundles being situated chiefly on the surface 

 of the strands, and the xylem, radially placed, forming plates proceeding from the 

 periphery towards the inside, between the phloem - bundles, as shown in the 

 accompanying figures. One 

 of the most striking pecu- 

 liarities of the root-strand is 

 that the narrowest, thinnest 

 vessels lie on the outer cir- 

 cumference, and are those 

 first formed : further towards 

 the interior of the strand, 

 vessels with continually 

 wider lumina become de- 

 veloped. Only when the 

 root-strand possesses amore 

 considerable thickness, is 

 its axial space occupied by 

 a kind of pith — i. e. paren- 

 chymatous tissue : in very 

 thin root-strands this is en- 

 tirely wanting, and, indeed, 

 a vessel, or a group of 

 vessels may run in the axis 

 of the strand. Moreover, 

 between the vascular plates 

 and phloem -bundles are 



formed small quantities of parenchymatous tissue, filling the interspaces to a certain 

 extent; as in the bundles of the shoot, parenchymatous elements are present in 

 addition to the vascular and fibrous elements. 



If we now, finally, take into consideration the cell-forms of the vascular bundles 

 themselves, already mentioned, the vascular elements are most prominent in both 

 parts. Those in the xylem are generally termed, shortly, vessels ; while those of the 

 phloem, corresponding to them, are named sieve-tubes. 



As vessels in the wider sense, and thus comprehending both the forms men- 

 tioned, are to be distinguished longitudinal rows of cells, the cavities of which 

 communicate directly with pne another by the transverse septa being either com- 

 pletely absorbed, or perforated by pores. 



Fig. 140.— Root of Acorus Calamus. Transverse section of the axial cylinder 

 with surrounding cortical tissue, s Endodermis ; /, p narrow, peripheral (oldest) ves- 

 sels ; ^ wide, inner (younger) vessels ; ph phloem. 



