small cavities, through which the ray lines are continued. The rest of the area is 

 mainly parenchymatous, grading to collenchymatous tracts at upper and lower surfaces, 

 as cells with few chloroplasts and few starch-grains. Stomata are rare : the green 

 mesophyll is continued over the upper surface, below the collenchyma, but without the 

 palisade-effect. The lower parenchymatous tract of thin-walled large cells, up to 90 

 //, diam., is marked with radiating bands of thick-walled, but non-lignified, fibres, up 

 to 30 //, diam.: there is no special cambium; but a good effect of radial rows at the 

 junction of x an< ^ </>> tne smallest cells 10 /* by 5. A zone of large rounded cells 

 (especially in sp. mat., in which they go brown as a tannin-effect) may be partially 

 isolated as an endodermis on the flanks of the strand. 



Shaded leaves differ in smaller size, less differentiated palisade, and little or no 

 starch-content. 



Other leaves may be compared, taking more particularly stout forms of evergreen 

 habit, as available fresh at any time of the year, as : 



Privet (Ligustruni) with good differentiation of palisade, and abundant stomata 

 on lower surface ; but the spongy mesophyll is less marked and the intercellular spaces 

 are scanty. 



Box (Buxus) with a very dense palisade-region and almost colourless lower half; 

 thick cuticle, prominent V.B. and good stomata. 



Ivy (Hedera) less differentiated mesophyll, palisade very poor in shade-leaves, 

 abundant stomata. 



Aucuba (yellow-spotted variety), ill-differentiated mesophyll and midrib ; chloro- 

 phyll, as also starch, conspicuously wanting in the yellow areas : variegation implies 

 malnutrition. 



Holly (Ilex) presents more marked differentiation, but along similar lines: 

 palisade in 3 tiers, covered by aqueous hypoderm, as well as aqueous epidermis. Arcs 

 of fibres over the phloem of V.B. Elaborate spongy mesophyll, and good stomata on 

 lower surface. Cuticularized layer of upper epidermis 10 //, thick. 



Stomata in surface-view are readily examined by skinning the epidermis of 

 leaves, as : 



Lilium candidum, large stomata 65 /* long and 50 broad, all orientated along 

 the long axis of the leaf, on lower surface only: guard-cells with conspicuously 

 abundant chloroplasts: slit to 20 ^ wide, as measured by air-bubble in water-preparation. 



Arum italicum, fresh leaves afford admirable example of mosaic epidermis; 

 stomata numerous, 100 per sq. mm. ; guard-cells 50 p. long, with subsidiary cells on 

 either flank. Slit opens to 15 /x; closed stomata show guard-cells straightened out 

 and closely approximated. Iodine sol. gives purple-red or violet colour to amyloid 

 material soluble in cell-sap of ordinary epidermal cells only ; the cytoplasm and well- 

 defined nuclei (15 //,) colour yellow. The guard-cells have no amyloid, but small 

 chloroplasts and starch-grains. 



Iris, stomata about 120 per sq. mm., on both sides of leaf, all orientated in 

 longitudinal direction with elongated epidermal cells : guard-cells 50 //, long, slit opens 

 10 /u.. Other leaves as available, cf. Tulipa, Hyacinlhus, Scmpervivum. 



Leaf-arrangement: Leaves arise at the growing-points of the stem only, in 

 strict acropetal order, as outgrowths of the meristem, with definite arrangement, more 

 or less a constant for different plants ; in rhythmic sequence, so working out a definite 

 pattern (phyllotaxis) ; normally spiral, or whorled. Points of insertion termed nodes ; 

 spaces elongated between as inlernodes. Whorled systems give 2-3 or more leaves 

 spaced at same level, and alternating at successive nodes; e.g. (2 + 2) system = 

 decussate, a common case ; cf. Privet, Box, Ash, Sycamore, Aucuba, Fuchsia, &c. 



In spiral constructions, successive members are spaced around the growing-point 

 at about 137^ (Fibonacci angle), which gives optimum balanced effect, as also 

 maximum exposure to light. A pattern is worked out in which contact-lines may be 

 traced (2:3:5:8:13:21, &c.) according to the size of the members produced, as seen 

 in transverse sect, of the young bud. Low ratio (2 : 3) in Ivy, Holly, Cherry Laurel, 

 &c. : for better examples, cf. Pine-cone (5:8), or Sunflower-head (34 : 55), and even 

 (89:144). 



B 



