114 
MORPHOLOGY OF TISSUES. 
the stem or root grows. In Dicotyledons, on the contrary, after the first year, 
a combination of vessels and wood-prosenchyma, often mixed with wood-paren- 
chyma, is annually formed. In trees with annual rings in the wood a periodicity 
may be remarked in the development of the cells of the xylem ; and on this depends 
its stratification into annual layers. Not unfrequently the phloem-portion also shows 
a similar stratification. In the closed bundles of Monocotyledons the order of 
development in the first year is similar to that already described. In Fig. 92, for 
example, the annular vessel r is first formed in the xylem-portion, then the spiral 
vessel s, then, advancing right and left, the pitted vessels gg, and in the middle 
(advancing radially) the narrow pitted vessels. It sometimes occurs {e. g. in 
Calodracon, according to Nageli) that the formation of vessels advancing right 
and left encloses the procambium, which afterwards passes over into latticed cells. 
Fig. 95. — Long-itudinal section of a fibro-vascular bundle of Ricinus, the transverse section being shown in Fig. 93; 
r cortical parenchyma; gs bundle-sheath; i)i parenchyma of the pith; b bast-fibres; / phloem-parenchyma; c cambium, the 
row of cells between c and / develops afterwards into a sieve-tube. In the xylem-portion of the bundle the elements are 
developed from j successively to t' ; s the first narrow and very long spiral vessel, j' wide spiral vessel, both with a spiral 
band which can be unrolled ; / vessel thickened partly in a scalariform, partly in a reticulate manner ; h h' wood-cells ; t pitted 
i vessel, at q the absorbed septum ; h" h'" wood-cells ; t' pitted vessel, still young ; the pits at first show the outer border ; 
afterwards the formation of the inner orifice commences ; 1 1 f in the wall of the vessel are observed the boundary-lines of 
the adjoining cells which have been removed. 
In the petiole of Pieris aquilina the development of the xylem begins in the 
procambium-bundles, by the formation of some narrow spiral vessels in the foci 
of their elliptical section ; scalariform vessels are then formed in the direction of 
the longer axis of the ellipse, first centrifugally then centripetally, until a compact 
woody mass is produced, elongated in transverse section; round this the pro- 
cambium which is still left is transformed into latticed cells, sieve-tubes, and 
cambiform tissue, and partly (at the circumference) into bast-fibres (Figs. 94, 96). 
The same is the case with most concentric bundles of Cryptogams. 
T:he Fibro-'vascular System of Roots. Bundles of the kind now described traverse 
the stem and branches usually in large numbers (sometimes, as in Palms, the number 
is enormous), bending at their upper end into the leaves, where they ramify copiously in 
