BUNDLES OF THE STEM. 



123 



acter of the latter and the softer texture of the former. As has 

 been before noticed (see 345), in dicotyledons and gymnosperms 

 in which there is annual increase in diameter there is a layer 

 of peculiar merismatic tissue (cambium) between the two parts. 

 It is generally easy to identify the cells of this cambium layer, on 

 account of their elongated form and intimate contact with each 



other. Their development gives rise (1) to new cells like them- 

 selves, (2) to cribrose and (3) to woody elements ; all of which 

 are to be examined later, under " Secondary Structure." 



370. The sheaths of collateral bundles may have the character 

 of typical endodermis and envelop the single bundles, or may 

 consist of strands of long fibres (hard bast), which are on one 

 side of the cinbrose portion, and accompany the bundle through 

 its whole course in the stem. The strands of fibres frequently 

 encroach upon the cribrose part of the bundle so much as to be 

 more or less commingled with it (see 311). 



371. The stem may sometimes have bicollateral bundles either 

 (1) with the woody part on the interior as well as on the exterior 

 aspect (e. g., Cucurbitaceae), or (2) with an envelope of wood 

 surrounding the liber ; this envelope is seen at the extremities of 

 the bundle, while the rest of it has the ordinary character (Iris). 



372. The bundles of the stem may be concentric (see 313) ; a 



FIG. 100. Longitudinal section of a fibro-vascular bundle of Ricinus (the cross-section 

 being shown in Fig. 99): r, cortical parenchyma; gs, bundle-sheath; 6, bast-fibres; 

 p, phloem-parenchyma; c, cambium (the row of cells between c and p develops after- 

 wards into a cribrose-tube) ; in the xylem portion of the bundle the elements are devel- 

 oped successively from to t' ; , the first slender and long spiral duct; s', wide spiral 

 duct, the spiral band uncoiling ; I, duct, thickened partly in a scalariform, partly in 

 a reticulate manner; h, h', h", h'", wood-cells; t, pitted duct; </, absorbed septum; 

 t', pitted duct, still young; in /, t, And t' the boundary lines of the adjoining cells which 

 have been removed are shown in the wall of the ducts; m, parenchyma of the pith. 

 (Sachs.) 



