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 Sy 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 

 CalodracoTiy according to Nageli) that the formation of vessels advancing right 

 and left encloses the procambium, which afterwards passes over into latticed cells. 



Fig. 95. — Longitudinal section of a fibro-vascular bundle of Riciiius, the transverse section being shown in Fig. 93 ; 

 r cortical parenchyma; gs bundle-sheath; m 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 s successively to t' ; s the first narrow and very long spiral vessel, s' 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 

 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 ; a.t £ 1 1' in the wall of the vessel are observed the boundary-lines of 

 the adjoining cells which have been removed. 



In the petiole of Pteris 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. 



^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 



