172 



APPLIED BIOLOGY 



FIG. 48. Half of transverse section of 

 dicotyledon stem (Aristolochia). p, 

 pith (white background of figure) ; 

 seven fibro-vascular bundles (v) ; cam- 

 bium (c) ; e, epidermis. All outside 

 the cambium is bark, its middle region 

 (6) is composed of hard bast-fibers. 

 (Modified, from Strasburger.) 



Some of the fibers found in the 

 bark are sieve-tubes, which 

 have already been mentioned 

 (103) as the path of sap down 

 stems. 



164. Microscopic Structure 

 of Dicotyledon Stems. (D) Ex- 

 amine one-, two- and three-year 

 stems in very thin transverse 

 sections. Identify the parts 

 which can be seen without a 

 microscope, and then note the 

 appearance of the cells compos- 

 ing them. Compare sections 

 seen with the microscope and 

 Fig. 48. A cross section of a 

 very young dicotyledon stem (e.g., bean or geranium) consists of 

 thin-walled cells (pith-cells) and fibro-vascular bundles ; but the 

 bundles, instead of being scattered irregularly as in corn stem (Fig. 

 44), are arranged in a circle (Fig. 

 48). In the very young stem the 

 bundles in the circle are widely 

 separated by many pith-cells ; but 

 soon new bundles appear between 

 the old ones, thus completing the 

 circle and forcing the pith into thin 

 plates (pith-rays or medullary rays) 

 lying between the bundles (Fig. 

 46). If a single bundle be exam- 

 ined, it will be seen to consist, as 

 in the corn , of wood-cells and large 

 tubes on the inner side, and sieve- 

 tubes and wood- or bast-cells on 

 the outer side (Fig. 49) ; but un- 

 like the monocotyledon (corn) 

 bundle, the dicotyledon bundle has 

 between these two regions a layer FIG. 49. 

 of cells (cambium) which are ca- 

 pable of dividing, forming new 

 bark (consisting of bast- and sieve- 

 tubes) on the outer side and new 

 wood-cells and tubes on the inner 



Transverse section of vas- 

 cular bundle of a dicotyledon. 

 Lower part of figure is towards 

 center of stem. /, woody fibers 

 which surround the bundle; s, 

 sieve-tubes mingled with other cells; 

 d, wood-ducts; c, cambium layer. 



