for all correcting curvatures. Adult differentiation of primary tissues is rapid ; the 

 first expression of tissue-differentiation being seen in the initiation of protoxylem 

 tracheides at usually only a few points. Hence roots are described in terms of 

 protoxylems as ' diarch ', triarch, tetrarch, &c., and ' polyarch ' when the number is not 

 obvious at a glance. Maximum number in large Monocot. types may be 100 or more. 

 The limiting case of centric symmetry is diarch ; and this becomes the commonest 

 case for all small roots. Root-structure also shows a characteristic arrangement of 

 the xylem, possibly as retention of an older mode of organization; the xylems 

 alternate with the phloems on different radii (' Radial ' arrangement, as opposed to 

 ' collateral ' in the stem), and the xylem units differentiate centripetally ; the first-formed 

 tracheide being external in the stele, abutting on the pericycle, and hence termed 

 exarch, as opposed to endarch and centrifugal in the stem. 



RANUNCULUS repens (Buttercup): The general scheme of primary tissue 

 organization may be followed in stout roots, 2 mm. diam., in which no secondary 

 complications obtain. More normal roots present secondary thickening by a cambium 

 at an early stage, and ultimately become woody axes, like old stems, in which 

 conduction alone remains ; absorption being restricted to the growing tips penetrating 

 new ground. In transv. sect, note : 



Cortex: wholly parenchymatous, of rounded cells (50-100 ^ diam.), with 

 intercellular spaces, and storing abundant starch-grains (12 //,); bounded externally by 

 Piliferous layer, of simple papillose cells, 25/1,, without root-hairs, and Exodermis of 

 thin walls, no special contents, and closely approximated radial walls ; internally by 

 Endodermis of oval cells (25 /x), radial walls with 'dot '-effect, owing to chemical 

 alteration of walls at point of contact. 



In older roots the cortex becomes ragged and lacunar, the endodermal cells 

 lignify at the dot, add a thickening layer, and this in turn is lignified. 



Stele, circular, (300 //, diam.) conveniently fills field of high power. 



Type tetrarch (varying 3-5), with 4 protoxylems symmetrically spaced, alternating 

 with 4 phloem tracts. The former continue differentiation to meet in the centre, thus 

 leaving no medulla. In young roots only the first spiral tracheides are in evidence, 

 and the others as thin-walled dilatations ; in older roots fully differentiated pitted- 

 vessels extend to the centre, and sclerosis may extend to conjunctive tissue on the 

 flanks of the xylem. 



The Pericycle (as the region between the protoxylem and the endodermis) is 

 reduced to the limit of one row of small units (12-25 //,). 



The Protoxylem units as spiral tracheides are small (12 /A), and a large pitted 

 vessel (75 /A) may occupy the centre of the stele : the entire xylem-tract is limited to 

 about half a dozen tracheides. 



The Phloem tracts are vaguely oval regions between the xylems, of a few small 

 parenchymatous units (12 /x), with few (1-2) sieve-tubes (20 //,) with companion cells. 

 A zone of small-celled conjunctive parenchyma separates the $ from the x> and follows 

 round internally. 



In older roots definite indications of tangential division in some of the cells of 

 the innermost phloem express the initiation of cambtal mechanism, but get no further. 



Deposits of starch in the cortex express the utilization of these roots for storage 

 of food-reserves over the winter ; these are conveniently cleared by the use of potash ; 

 the tissues are well differentiated by Phloroglucin-H 2 SO 4 , or wholly cleared in Eau 

 de Javelle. 



Special Cases : The roots of water-plants, attached in mud, show extremely 

 deteriorated organization ; root-hairs are wanting, and vascular tissues are reduced to 

 a minimum. Many epiphytic orchids, of tropical rain-forests, and in greenhouse- 

 cultivation, send out aerial-roots, in which a peripheral tissue, delimited by an exodermis, 

 and of varying thickness, is specialized as dead units with tracheidal thickenings, which 

 store rain-water, as a spongy system, Velamen : conduction is effected by living 

 ' passage-cells ' distributed in the exodermis, and storage-tracheides occur in the cortex. 

 The stele is of the polyarch Monocot. type. 



Primary structure is also well shown in the primary roots of seedlings of 

 Phaseolus (Kidney Bean), about 2 in. from the tip. The tissues are soft, but the 

 radial dot of the endodermis is particularly well-defined. 



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