radial divisions to add new rows. The term covers the initial cell of each row, 

 as also, more loosely, all units still small and not obviously differentiated. 



(9) The Xylem-region internal to the cambium shows at first thin-walled units in 



process of elaboration ; vessels dilating at the expense of adjacent units before 

 acquiring the special wall-thickening, and retaining p. contents and nucleus. 

 The fully differentiated region, picked out by the phloroglucin-reaction, is ligni- 

 fied, and consists of: 



I. Metaxylem of 



(a) Pitted Vessels, appearing as large oval or rounded empty spaces, to 100 ^ 

 or more diam., wall thick, with beaded effect due to closely set small bor- 

 dered pits (5 //), deeply lignified ; between these 



(t>) Xylem Fibres, as units similar to the fibres of the Pericycle, lignified, with 

 thick walls and slit-pits, closely packed by mutual pressure. 



(c) Xylem Parenchyma, lignified, with conspicuous pits on end walls. 



II. Protoxylem of rounded tracheides, in more or less radial series, diminishing in- 



wards, often in finger-like series, including 



(a) Spiral Tracheides, to 70 /z diam., showing cut end of a spiral thread-thicken- 

 ing : 3-4 in series, to smaller ones of 40 /*. 



() Annular Tracheides, the smallest, innermost, lignified units, with no cut 

 thread (i. e. ring-thickening). Surrounding these 



(c) Xylem Parenchyma, as small-celled undifferentiated tissue, ultimately 

 sclerosed. 



(10) Medulla of larger parenchymatous cells, as also in Primary Medullary Rays, 



20-60 fj. diam. In the centre of the pith, which soon dies, and contains air 

 only, cells distend to 300 p diam. 



Note. The connexion of Interfascicular Cambium across the rays by the inner 

 pericycle; differentiation of Metaxylem takes place centrifugally, that of Phloem 

 centripetally. New vascular strands have no ' proto '-regions. The Protoxylems 

 differentiate centrifugally (' endarch ') : the vascular tissues will ultimately complete 

 the circle as a ' vascular cylinder '. On the flanks of the bundles pitted-tracheides 

 occur, intermediate to vessels, as at the periphery of the protoxylem reticulate tracheides 

 are intermediate to simpler spiral units. 



Longitudinal section, cut radially through one V.B., shows general extension 

 of all the tissues more or less : isodiametric cortical parenchyma may be twice as long 

 as wide ; endodermal cells 3-4 times. The fibres are apparently indefinitely long, 

 with attenuated ends, and the slit-pits show on the face-walls. Sieve-Tubes, 200 /z 

 long, with sieve-plates obscurely perforated and 20 \L diam. ; the coagulated proteid 

 contents (and slight starch) staining deeply with Iodine-sol. Cambium units as 

 delicate longitudinally extended cells (200^), with thin walls, granular contents and 

 elongated nuclei. Pitted Vessels, formed of segments 300 p long, or more, with 

 close-set oval, eye-like, bordered pits (6 /x). Xylem fibres as slender units with pointed 

 ends, small slit-pits, both ends not seen ; mingled irregularly in metaxylem. On the 

 flanks of the bundle, pitted tracheides come next to pitted parenchyma of the M. rays, 

 rectangular cells, 100 //, or more long, with pitted walls. 



Protoxylem of tracheides, variously thickened ; as reticulate, with broad meshes, 

 grading to pitted forms, 300-400 //, long; spiral tracheides with spiral thread (i or 2), 

 running distinct, anastomosing, or branching, pulled out more or less according to 

 age ; the thread alone lignified. Annular tracheides as the innermost, with perfect ring- 

 thickening alone lignified, and pulled out to ioo/x, apart. Transitions to spiral also 

 occur. 



Iodine-solution is the best differentiating reagent for cell-contents; Phloroglucin- 

 2 5 % H 2 SO 4 for differentiation of lignified walls ; Chlor. Zn. lod. is often feeble for 

 spirit-material, though admirable for fresh : In extreme cases Iodine sol. and 66 % 

 H 2 SO 4 gives a deep blue with celluloses, but is apt to decompose delicate tissues. 



Drawings of such an indefinite complex, no two sections being alike, are 

 preferably built up by taking one unit of each kind (or two), in succession, as 

 a composite figure including all cell-details, rather than a fancy sketch of the field of 

 view : In the case of pitted tissues of different order, note the detail on either side of 

 the party-wall, as seen by focussing the cut edges. 



