protoxylem of ill-differentiated spiral tracheides extends to a broad tract of small 

 celled (20 fj) parenchyma. Internal to the latter, a semicircular arc of Medullary 

 Phloem repeats the organization of the outer phloem, but is not capable of extension 

 by a cambium. 



In Longitudinal sect, note, the large sieve-tubes of the primary (outer) 

 phloem, 600-750 /x long, and 50-75 /x wide, with sieve-plates as transverse septa, 

 5-6 ^ thick, and clearly perforated. In sp. mat. the coagulum of proteid may be 

 squeezed through the pores, and take the impression of the sieve. Pitted areas on 

 the side-walls communicate with the companion-cells, as lateral ' sieves '. In older 

 parts these are all blocked with callus, as carbohydrate waste of proteid-conduction, 

 30 /x thick on the main sieve, and as thick masses on the laterals. Later sieve-tubes 

 from the cambium are shorter, 200 /x; companion-cells correspond in length to their 

 sister sieve-tube: young sieve-tubes show nuclei 30 /x diam., but these disappear later. 

 Cambium cells are 150-200 /X long: the Pitted Vessels are built up from segments 

 100-200 fji long, all stages being found ; the bordered pits are in the meshes of a fine 

 reticular thickening. 



ZEA (Maize) as a strong Grass, affords a convenient example of the Monocot. 

 type of construction : the stem is of annual duration, with well-marked nodes and 

 internodes ; the latter with considerable capacity for intercalary growth at basal region. 

 Avoiding pieces too hard, with silicified epidermal wall, transv. sect, of a young 

 growing stem (10-15 mm - diam.) shows V.B. scattered somewhat irregularly over 

 the section, about 500, of which f are more peripheral ; each V.B. consists of x an d <, 

 but with no cambium, and no secondary increase. The general scheme of preceding 

 types appears at first sight wholly wanting, but fibres are more numerous in the 

 peripheral region with smaller bundle-ends, and each V.B. presents normal orientation ; 

 the best V.B. are those nearer the centre. Individual V.B. are small tracts 2oo/x 

 (100-300) diam., all on the same plan ; one may be taken in detail : 



Two large pitted- vessels placed laterally (60 /x) are most conspicuous ; as also 1-2 

 smaller ones in the radial line, 50-30 p, as roughly a V-shaped group, the apex of 

 which is occupied by an intercellular lacuna, and the fork subtends a small oval 

 phloem tract (100 /x). The innermost tracheide is annular (often only as suspended 

 loose rings), the next is spiral, and between the large p. v. a series of pitted-tracheides, 

 56 wide, closes the gap ; while small cells of xylem parenchyma fill the space around 

 the spiral tracheide. The Phloem consists of sieve-tubes (15 /x), as the larger units 

 of the tract, more or less in a chess-board alternation with smaller cells (6 JM), 

 apparently functioning as companion cells, with marked contents. Crushed relics of 

 Protophloem appear at the limit of the V.B., pressed against an investing sheath of 

 sclerosed fibres (1-2 deep) which more or less surrounds each bundle, being greatly 

 exaggerated in the cortical region of the stem. Ground-tissue of thin-walled 

 parenchyma, to 150-200 /x, with marked aeration, fills the intermediate regions of 

 the stem. 



In Longit. Sect, different units may be checked by their position in transverse 

 scheme. The pitted-vessels are made up of segments 600 /x long, with close set oval 

 bordered pits, 6 /x; the annular vessels with hoops 50 /x diam., spaced as much apart 

 or more, are the finest elements of this stem. The sieve-tubes are feebly differentiated, 

 with sieves hardly noticeable. Note general absence of starch, tannin, and crystals. 

 Cellulose tissues are differentiated by Iodine and 66 % H 2 SO 4 . 



The biological organization of the stem is based on strength and flexibility rather 

 than rigidity, and this is secured by a very uniform distribution of fibrous tissues 

 associated with the vascular strands. 



H IPPU R IS (Mare's Tail) ; Dicot. aquatic type with greatly deteriorated conduc- 

 tion system. In transv. sect, of internode, 5-6 mm. diam., Note central stele, circular, 

 i mm. diam. or more, and cortex with large lacunae ( to i mm.) separated by plates of 

 cells, i deep, giving great internal atmosphere. 



Epidermis of small cells with thin cuticle ; Endodermis conspicuous, cells oval 

 (40 /x), with starch-grains, and ' radial dot '. No differentiation of V.B., no cambiumj 

 no M. Rays, no mechanical fibres, no distinct pericycle : a zone of ill-differentiated 

 phloem of small cells, largest (sieve-tubes) 20 /x diam., surrounding Xylem of reticulate 

 and spiral tracheides (30x1), mingled with small-celled parenchyma. Broad central 

 parenchymatous medulla. A trace of cambial division outside xylem in older stems. 



16 



