Structural Botany : Organization of the Herbaceous Stem. VI 



The function of the stem is to support, space out, and display the photo- 

 synthetic leaf-members, to supply them with food-salts, and act as the general path of 

 conduction from the absorbing root. Hence its tissues are differentiated in connexion 

 with (i) Conduction of water and solutes to the lamina ; (2) conduction of elaborated 

 food-material to other growing but non-photosynthetic regions ; (3) mechanical sup- 

 port and ramification over a considerable space- dimension ; (4) in lesser degree for 

 storage of reserves. 



The tissues concerned with these functions are respectively : (i) Xylem, (2) 

 Phloem, (3) Mechanical tissues (Stereome), more particularly as sclerenchymatous 

 fibres, to a lesser extent Collenchyma in young axes, (4) all living parenchymatous 

 units. 



Ramification of the shoot is normally provided for by the initiation of new grow- 

 ing-points in the axils of the leaf-members, hence termed axillary-buds ; the limiting 

 case of one such bud being normal for all foliage-leaves. 



A good example is afforded by the stem of the great Sunflower : 



HELIANTHUS ANNUUS, which may grow 12 ft. high, and 2-3 in. diam., 

 in the course of a few months. Small stems, J in. diam., from an actively growing 

 plant are most convenient. Hard stems and soft ends may be rejected. In transv. 

 sect, the bundles are visible to the eye, as an enlargement of the ^ in. stem : about 

 20-30 are evenly distributed around the periphery, about 2 mm. from the margin, and 

 the centre includes a large pith, 9 mm. diam. Section need not include more than 

 3 bundles. Iodine sol. gives immediate differentiation of the V.B. masses, endoder- 

 mis with starch grains as before, tracts of scl. fibres (yellow) in the pericycle external 

 to the V.B., as if associated with them : a definite cambium-zone passes from bundle 

 to bundle. The cortex shows an outer collenchymatous region and a few ducts. 

 The bundles vary in size, some very small. Select a medium well-differentiated one 

 for more detailed examination. Tissues from the periphery inwards include : 



(1) Epidermis, of oval cells, 30 /x wide, with aqueous contents, slight pitted areas, 



thicker outer wall, and distinct cuticle. 



(2) Collenchyma, of a dozen rows or more; cells 15-20^, oval, with cellulose 



thickening at angles and on tangential walls ; scanty contents with a few chloro- 

 plasts ; grading into : 



(3) Cortical Parenchyma, of thin-walled cells, oval, up to 100 /u wide, with scanty 



contents ; many in active division by radial walls. Occasional resin-ducts show 

 a cavity, 20 p. diam., surrounded by a series of a dozen or more small, 20 \*. 

 secreting cells, grading into ordinary parenchyma. 



(4) Endodermis, as a wavy line of cells, in single series, pressed outwards opposite 



the V.B., and dipping between them ; cells to 50 //, more or less with starch- 

 grains. Radial walls in close contact. 



(5) Pericyclic Fibres ; opposite each primary V.B. an oval tract of very distinct 



tissue, as closely packed units (up to 500 or so) with minute intercellular spaces 

 only ; walls thickened, with slit-pits, and deeply lignified (giving phloroglucin 

 react.). Average diam. 25 n, but many smaller attenuated ends cut as the effect 

 of sliding growth. Little living contents left at this stage. 



(6) Pericyclic Parenchyma fills the regions between the fibrous tracts, cells of 



which may be also dividing by radial walls to keep pace with the growth of the 

 stem. 



(7) The Phloem, immediately within the fibrous tract, consists of units with cellu- 



lose walls and protoplasmic contents, differentiated into : 

 (a) Sieve-Tubes, 20-25 /* diam., with contents as dense coagulated proteid masses 



(or falling out in sect, of sp. mat.). 

 (6) Companion- Cells, as small units (6-8 /*) with dense granular contents, 



associated with the sieve-tubes, as at their angles. 



(c\ Phloem-parenchyma, as all cells not distinctly referable to the preceding. 

 (d) Protophloem, suggestions of small cells at the external limit of the tract. 



(8) The Cambium as radial rows of small cells, 15 ^ wide, actively dividing by 



tangential walls to build new tissues in both directions radially, with occasional 



13 



