Structural Botany : The Leaf. III. 



A well-grown leaf of CHERRY LAUREL (Prunus Lauro-cerasus) from 

 a sunny position, consists, of a broad lamina, 8 in. by 3, on a short stout petiole ( in.). 

 Note conventional usage of borrowed terms as midrib, more conspicuous on lower 

 surface ; apex attenuated as a drip-tip ; veins (nerves) as paths of conduction ; teeth of 

 margin, and especially dorsiventrality as implying bilateral construction with two sides 

 unlike, the distinction of D.V. surfaces being purely arbitrary. 



Orientation on plant in spiral phyllotaxis, with lamina presented to maximum 

 illumination, the upper surface at right angles to direction of incident light; by 

 secondary twisting of petiole giving 2 rows ; more or less wanting in shaded shoots. 



Venation seen particularly well on lower side, pinnate, in alternating sequence ; 

 with expressions of 'triple-arch' system towards margin, as mechanical adaptation 

 against shearing action of wind. Obscure glands (hydathodes) paired at base of 

 lowest lateral veins. 



Transverse section of sample of midrib and lamina of fresh leaf (half-way up), 

 cut in pith, mounted in water : note shape of midrib, central vascular strand and little 

 green tissue ; lamina of uniform thickness (-| mm.) chiefly of green tissue (mesophyll) 

 with a few obliquely cut portions of small V.B. as ' veins '. Taking the tissues in 

 order from upper surface to lower : 



(1) Upper Epidermis of cells fairly isodiametric (30-60 /* broad and 30 high), 

 with stout cellulose walls, simple pits, no chloroplasts, slight aqueous contents ; with 

 distinct cuticle on outer surface, 3 p thick ; differentiated by Chlor. Zn. lod. as blue 

 cellulose, yellow cuticle, and brown intermediate cuticularized layers. 



(2) Palisade Mesophyll as 2 tiers of columnar cells set at right angles to 

 surface, averaging 50 /z deep each, and 18 p, wide; occasional attempts at 3 tiers; 

 shade-leaves have broader more rounded units. Walls thin, of cellulose ; contents 

 include discoid chloroplasts (3-6 /*) with small starch-grains (tested by Iodine sol.). 

 Note the manner in which the units of the basal tier are fitted on the subjacent 

 parenchyma. 



(3) Spongy Mesophyll, a lower zone, 200 /* deep, of loose isodiametric 

 parenchymatous cells, with chloroplasts and starch in smaller quantity ; up to 75 \j. diam., 

 with greatly exaggerated intercellular spaces, but often irregularly pulled out giving 

 stellate effects. Chloroplasts with included starch-granules, being fewer, are seen to be 

 more distinctly restricted to a parietal layer in each cell. Note the netted effect of 

 the sponge-like type of spacing for effective aeration. Both palisade and spongy 

 mesophyll may contain large crystal-aggregates of calcium oxalate (sol. in dil. HC1) ; 

 crystal-sacs contain one stellate cluster (35 p), more general in palisade of older leaves, 

 or large single rhombs (45 /x), especially in spongy mesophyll. 



(4) Vascular Bundles, in smaller veins, cut more or less obliquely, with 

 a distinct sheath of parenchyma (endodermis), and poorly differentiated spiral tracheides 

 (giving phloroglucin -H 2 SO 4 reaction), constantly on the upper side of the strand. 



(5) Lower Epidermis, similar to upper, but cells smaller and more rounded ; 

 similar aqueous contents and cuticle, with the additions of 



(6) Stomata, as slits communicating with the exterior, bounded by 2 guard-cells. 

 These are not well shown in Cherry Laurel, being obliquely cut; more distinct on 

 sloping side of midrib. Guard-cells with comparatively thick inner and outer walls, 

 outer pore-chamber, and cuticle following the contour of the guard-cell its whole course. 

 Irregularities in the epidermis, due to stomata being cut obliquely are more evident in 

 Chlor. Zn. lod. preparations. Stomata are readily seen in surface- view by skinning 

 off a small piece of lower epidermis and mounting in Iodine sol. There may be 200 

 per sq. mm., each 40 /x by 35 across the guard-cells when widely open. 



Note the mosiac effect of lower epidermal cells with simple pit-communications 

 (giving beaded effect to walls). The guard-cells contain small chloroplasts, small 

 included starch-grains, and a central nucleus. 



(7) Midrib, appearing as a broad area of generalized structure, bilateral and 

 eccentric ; central vascular strand as conspicuous band of Xylem of pitted tracheides 

 and vessels (to 30 /* diam.), with radiating bands of parenchyma as ' medullary rays ', 

 differentiated by Chlor. Zn. lod.; tracheides give red reaction with phloroglucin -H 2 SO 4 . 

 On the lower side of the Xylem is a narrow band of phloem, with cellulose walls, and 



