Vitis pterophora, a greenhouse climber, which drops adventitious air-roots for 

 several feet, in lengths with clean surface and very uniform structure. 



(1) Softer young roots, 2 mm. diam., show well-marked protoxylems, 4 or more 

 (7), and conspicuous phloem-tracts of small cells, with distinct medulla and broad 

 cortex. The endodermis is thin-walled, and only distinguished by being the first row 

 of cells without intercellular spaces. Conjunctive parenchyma on the flanks of the 

 xylems may become sclerosed; but is distinguished by the different tint with 

 Phloroglucin-25% H 2 SO 4 . Root-hairs are wanting; the 2 layers subjacent to the 

 peripheral layer are distinct, and contain tannin. Protoxylem units grade up to 75 //, 

 diam., the first formed small and soon crushed. 



(2) A slightly older (firmer) stage shows cambial divisions freely produced along 

 the inner margin of the phloem tracts, giving distinct radial rows : large metaxylem 

 units are soon differentiated on the inner side of the cambium (90 /x) ; the phloem may 

 show tannin sacs ; a few cells of the pericycle become fibres, outside the phloems. An 

 active phellogen on the outside of the cortex gives cork-cells cutting off the peripheral 

 layers. Tangential divisions extend to the pericycle external to the protoxylems, and 

 the cambium is linked up, though distinctly 4-angled in outline in tetrarch roots. 



(3) Roots 2-| mm. diam. show a well-marked vascular cylinder with masses of 

 metaxylem alternating with the protoxylems, and large pitted vessels (ioo/x); but the 

 xylem is mainly parenchymatous: the tissue external 10 the protoxylem remains a broad 

 primary ray, and a secondary ray (often even broader) is initiated in the new metaxylem 

 tracts. The cambium zone is fully established, with particularly good radial rows. 

 Pericyclic fibres are followed up by fibres in the phloems ; tannin-sacs, raphides, and 

 cluster- crystals of calc. oxalate, add details of interest ; all parenchymatous units store 

 abundant starch. Thyloses are frequent. 



(4) In older roots, 4 mm. diam., with further increase, the large xylem vessels 

 (150/1) have fibres grouped around them : the protoxylems remain clearly defined, and 

 the tissues are clearly differentiated by Iodine, Potash, or Phloroglucin-H 2 SO 4 . 



Quercus (Oak) affords a good example with annual rings, in roots 3-15 mm. 

 diam. ; the protoxylems (6 or so) persist as a stellate group in the centre, and the 

 primary rays (broad rays) are clearly seen by the naked eye. The details of the 

 tissues follow those of the stem. 



Monocotyledonous type of root-construction typically differs in the complete 

 absence of secondary changes, and in polyarch organization. Succulent roots, 4 mm. 

 diam., of Asparagus present a wide cortex of rounded parenchymatous units (50-90 /x 

 diam.) with intercellular spaces. Stele, i mm. diam., the endodermis with U-thickening, 

 of small cells (12 /x). Pericycle reduced to one row (15 /x). Protoxylems about 20, 

 of very small tracheides (6 p) peripherally, but extending as a complete ring of vascular 

 tissue, with large p. v. up to 80 /x diam. Central medulla of 50 /x parenchyma. Phloem 

 tracts small, between the protoxylems, each with 2-3 sieve-tubes, and well-marked 

 cambiform units, apparently functional as companion-cells. The section contains no 

 starch, and clears admirably in Potash, or Phloroglucin-H 2 SO 4 . The peripheral layer 

 may have conspicuous root-hairs, but the subjacent tissue (6 rows or so) is thickened 

 and suberized. 



Orientation of the Boot is normally effected in terms of positive Geotropism 

 and positive Hydrotropism ; i. e. roots tend to grow downwards into the soil and to 

 ' seek water ', as part of their essential biological equipment. In lateral roots 

 hydrotropism is more pronounced ; the strongly-marked positive geotropism of 

 a primary sinking root is an adaptation to the same end of water-supply. Rarely, 

 in water-logged soils, roots may grow upwards to obtain free oxygen. Sensitivity is 

 more concentrated in the apical meristem (Zone I), and the mechanism of response, 

 by initiating a growth-curvature, is restricted to the zone of elongation, usually only 

 a few millimetres behind the apex. The very general occurrence of starch-grains in 

 the cells of the root-cap (not in the growing layers of the apex) has given support 

 (since 1900) to hypotheses of j/tf/tory/^-rnechanism, interpreted in terms of 'Falling 

 starch ', still largely speculative. 



Experimental observations are followed on the Clinostat, in which rotation on 

 a horizontal axis neutralizes gravity by giving equal presentation to the stimulus on 

 all sides. The effect of a continually acting force as 'g' may be imitated, or 

 replaced in intensified degree, by the utilization of ' centrifugal force '. 



22 



