THE STEM. 35 
Very young stems of most gymnosperms, ¢.g., Scotch fir and 
juniper, correspond 
in their main histo- 
logical features with 
those of dicotyledons 
(fig. 7, A, B). The 
most important dif- 
ference is found in 
the wood. This is 
almost entirely made 
up, not of vessels, 
but of elongated 
tapering elements, 
with pitted lignified 
walls and no proto- 
plasm, the internal 
cavity! being filled 
with air. Such air- 
containing cells are 
tracheides. | Wood 
vessels and trache- 
ides may collectively 
be called air tubes. 
The pits in the wall 
are somewhat pecu- 
liar. In - surface 
view they present 
the appearance of 
two concentric 
circles, and are 
therefore called bor- 
dered pits (fig. 7, C). 
The mouth of the 
pit, z.e., its opening 
into the cavity of 
the tracheide, is 
narrower than the 
bottom of the pit, 
and (the whole sec- 
tion being transpa- 
rent) it therefore 
appears as the inner 
circle, and the latter 
as the outer. In gym- 
nosperms, again, the 
sieve tubes are dif- 
(<75 
LO 
Pe, > 
siaesre 
LoS 
Fic. 7.—Minute Structure of Vegetative Organs [G, I, K, after 
Prantl}. All more or less magnified. A. cross-section of 
young shoot of Scotch fir. B. part of same enlarged ; ep. 
epidermis; p. pith; co. cortex; xz xylem; ph. phloém; 
ec. cambium; mr. medullary ray. C. small piece of radial 
section of fir wood, showing tracheides with bordered pits ; 
mr. medullary ray. D. cross-section of asparagus stem, 
showing bundles. E. one bundle of same, magnified ; 2. 
a’. xylem; ph. phloém. F. piece of lower epidermis from 
geranium leaf; g.c. guard-cells of stomata. G. cross- 
section of beech leaf; ep. upper epidermis; ep’. lower 
epidermis; pp. palisade parenchyma ; sp. spongy paren- 
chyma; st. stomata. H. part of cross-section of buttercup 
root; ep. epidermis; J. lateral root; gt. cortex; rh. 
root-hairs. I. section of elder lenticel; ep. epidermis; 
co. cork; 7. loose cork of lenticel; ¢.c. cork cambium. 
K. part of cross-section of four-year-old lime twig; p. 
pith; co. cortex; ¢. cork; ph. phloém; @, 1, 2, 3, 4. 
annual rings of xylem. 
