THE STEM CoOL 
the outer wall of which is slightly tickaned. This 1 is the 
dermatogen. : 
From these three primary layers of the growing point arise 
the three great permanent tissue systems of the adult stem. 
The plerome gives rise to the stele—that 1s, to everything 
within the cortex, the inner layer of which is called the 
bundle-sheath ; the periblem on each side gives rise to the 
cortex, whilst the dermatogen produces the epidermis. 
These may next be examined in a young Sunflower, which 
may be taken as a typical herbaceous stem of a mesophytic 
= plant. 
. Stem ee Cut the stem of a young Sunflower through ~ 
jy Sunflower. of the internodes, and observe with the naked eye. 
Fig. 18 represents a drawing made under a low power of the 
microscope. 
The most conspicuous part will be the white spongy pith in 
> the centre. This is parenchymatous tissue. Around. that, 
'~ arranged in a circle, is a 
. Yellowish - white,  fibrous- 
ae looking tissue. These are 
< the fibro-vascular bundles ; 
the spaces between are the 
hj medullary rays, and consist 
of parenchymatous tissue. 
\, Outside this ring of fibro- 
(2 Vascular tissue follows some 
: more parenchymatous tis- 
sue, forming the cortex of 
the stem, and outside of uo . 
all a skin, the epidermis. yg 1g,—Srem o¥ SUNFLOWER, 
Thus with the naked eye it TRANSVERSE SECTION. (Low power, ” 
Is possible to mak e out the ee cortex; f.v.t, Abts. 
three chief forms of tissue vascular tissue; 0, bast; w, wood; 
in stems, namely, the epi- m.r, medullary rays; p, pith; c, cam- 
bium; 7c, interfascicular cambium. 
dermis, the parenchyma, | 
and the fibro-vascular tissue. The minute structure is 
explained later. 

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