THE FIBRO-VASCULAR BUNDLES. 



97 



completed ; this occurs in particular with the open bundles of Dicotyledons and 

 Conifers. 



The development of the elements of a bundle always begins at single points 

 in the transverse section, and extends from them in different directions ; and thus 

 the permanent cells which arise one after another acquire different mature forms. 

 In the open bundles in the stem of Dicotyledons and Gymnosperms the development 

 usually begins with the thickening of single bast-cells on the peripheral side of the 

 bundle ; somewhat later single spiral vessels (or annular- vessels) arise next the pith ; 

 and while the development of the phloem proceeds centripetally, forming succes- 

 sively and often alternately bast-cells, latticed cells and parenchyma, — annular and 



UJ 



"-1-^^, 



r^ 



T!| 



Fig. 84.— Longitudinal section of the fibro-vascular bundle of Ricinus, the transverse section being shown in Fig. 82; 

 r cortical parenchyma ; g^s bundle-sheath ; tn parenchyma of the pith ; b bast-fibres ; / phloiim-parenchyma ; c cambium, 

 the row of cells between c and / is afterwards developed into a sieve-tube. In the xylem portion of the bundle the elements 

 are developed beginning from s gradually to ^ ; 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 ; 

 / pitted vessel ; at q the absorbed septum; h" h"' wood-cells; f pitted vessel, still young; the pits at first show the outer 

 border; afterwards the formation of the inner pore commences; at tt' t" in the wall of the vessel are observed the boundary- 

 lines of the adjoining cells which have been removed. 



spiral vessels^ either separately or together, or reticulated vessels and eventually 

 pitted vessels often alternating with wood-cells, arise centrifugally in the xylem 

 (Fig. 84). In Conifera? only prosenchyma-cells with bordered pits (together with 

 xylem-rays) are subsequently produced, so long as 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-parenchyma, is annually formed. In trees 

 with annual rings in the wood a periodicity may be remarked in the development 

 of the xylem-cells ; and on this depends the stratification of the xylem 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 



1 These are formed only before the completion of the increase in length of the organ, to which 

 the bundle belongs. 



