enunciation of three 'embryonic layers', as dermaiogen, peribhm, and plerome, 

 reflecting academic abstractions on the general lines of the germinal layers of the 

 higher animal (middle xix century). Such layering is often quite definite, but is 

 to be referred to a precocious differentiation of functional regions of the adult, 

 and is not causal. The terms survive as useful conventions and topographical 

 designations. 



Behind the growing-point, zones of differentiation may be distinguished as 

 (i) The apical zone of growth and division in all directions ; (2) A zone of elongation, 

 in which longitudinal extension is most active, giving a capacity for intercalary growth 

 and rapid increase in length; (3) A zone of differentiation, in which the tissues 

 assume their adult histological character and special physiological activities. 



HELIANTHUS, |in. stem: In a small herbaceous stem cells and tissues are 

 readily distinguished in transverse section, and may be tested by ordinary reagents : 

 e.g. examine a transv. sect, mounted in Iodine solution. The general circular 

 outline of the section is filled with more or less circular areas indicating the cell-walls, 

 grading from TO /A to 100 /A diam., with intercellular spaces between. Differentiation 

 of more specialized tracts is conspicuously marked at 12-15 spots, very uniformly 

 distributed about 300 /A from the periphery, as sections of the Vascular Bundles (V.B.). 

 A little distance external to these a distinct line of cells with black dots (starch- 

 grains stained by Iodine sol.) indicates the Endodermis. The whole section is 

 bounded by a single layer of cells as Epidermis, and special tracts may be 

 defined as: 



The tissue beneath the epidermis, as far as and including the endodermis 

 = Cortex: The tissue internal to the vascular bundles = Medulla (pith): tracts 

 between the V.B. are termed Primary (Medullary) Rays ; and a region between the 

 V.B. and the endodermis as pericycle. 



In further detail note : 



(1) The Epidermis of oval cells, 30 /A diam., with scanty granular contents, 

 thicker outer wall covered by a distinct cuticle ; and giving rise to outgrowths 

 occasionally as hairs of single series, or on multicellular bases. 



(2) Cortex of outer 3-4 layers of Collenchyma, the walls markedly thickened at 

 the angles (mechanically increasing cohesion of periphery). 



(3) Cortical Parenchyma of larger units, 40-100 /A diam., with occasional ducts. 



(4) Endodermis of a single line of cells, 50 /A wide, with starch-grains (5 /A) ; 

 in this stem curiously the only layer with starch. 



(5) The central Medulla, the bulk of the section, of rounded units, up to 120 /A 

 diam., with scanty contents and soon dying off. 



(6) The Vascular Bundles, differentiated by Phloroglucin -25 % H 2 SO 4 , into 

 a Xylem-region (x) of lignified tracheides, in 3-5 radial rows, to 40 /A diam., 

 surrounded by smaller non-lig. cells of 15 /A. : Phloem (</>) of small cells (15-20 /A) 

 with more definite contents : between x an d < the suggestion of a cambium-zone 

 of smallest cells (10-15 /*) 



(7) External to the phloem the beginning of a group of fibres, as a rule not yet 

 lignified, belonging to the pericycle, 30 /A diam. In older axes these have thick 

 lignified walls giving the Phloroglucin-reaction. 



Approximate details of measurement, freely interpolated, are not necessarily in- 

 tended to be checked, although it is a useful habit always to use a micrometer eye- 

 piece; so much as to afford a basis for accuracy in drawings, and keeping the 

 figures in their proper proportions. The most useful standardized drawings are those 

 constructed on the ' mu-millimetre ' scale (High Power), = x 1000, or in some simple 

 relation. 



Powerful reagents used for differentiating celluloses require great care. Alkalis 

 (KOH, NaCIO) spoil lenses, strong acids metal-work and clothes. Fuming acids 

 (HC1) should be avoided ; weak H 2 SO 4 with phloroglucin takes longer time, but does 

 not affect the intensity of the reaction. Preparations involving strong acid, or Chlor. 

 Zn. lod. should be put in a glass of water as soon as finished with, and not left lying 

 about. Small quantities only should be employed. 



