38 



EXOGENOUS OB DICOTYLEDONOUS STEM. 



the circumference. Immediately surrounding the pith is a layer of a 

 greenish hue, the medullary sheath, e m, from which the medullary 



rays, r m, proceed towards the cir- 

 cumference, dividing the vascular 

 circle into numerous compact seg- 

 ments, which consist of woody ves- 

 .'yp sels,/>, and of porous vessels, v p. 

 These are surrounded by a moist 

 layer of greenish cellular tissue, c, 

 J7 called the cambium layer, which 

 is covered by three layers of bark, 



ff^S^^P9^, ; -&%?" :x \i%fiP- P J c i e c i anc * Pi wrtn laticilerous 

 * ^ - vessels, v I, the whole being en- 



closed by the epidermis, e p. On 

 making a thin vertical section of a 

 96 portion of the same branch, and 



viewing it under the microscope, the parts composing the different por- 

 tions become more obvious 

 (fig. 9 7). The pith, m, with its 

 hexagonal cells decreasing 

 in size outwards, surrounded 

 by a narrow fibro- vascular 

 zone; the medullary sheath, 

 consisting chiefly of spiral 

 vessels, t; the medullary 

 ray, r m; the vascular zone, 

 consisting of porous vessels, 

 v p, of large diameter, and 

 , forming the large round 



fl fl & W apertures seen in a trans- 

 97 verse section; the woody 



fibres, //, with their thick walls and smaller apertures; the inner bark 

 or liber, f c, with the layer of cambium cells, c; the second layer 

 of bark, or the cellular envelope, e c, with the laticiferous vessels, v I; 

 the outer or suberous layer of bark, p, with the thin layer of epider- 

 mis, e p, having hairs scattered over its surface. 



74. Such is the structure of a young shoot during the first year of 

 its growth. At the end of a second year the shoot is found to have 

 increased in diameter by the formation of a zone of vessels consisting 

 of porous and woody tissue, and a zone of fibrous bark, the medullary 



\vl\c 



/* 



rr 



Fig. 96. Horizontal section of young stem of Acer campestre, magnified twenty-six diameters- 

 m. Pith, em, em, Medullary sheath. fb,fb. Woody bundles, v p, Porous vessels, r m, Medul- 

 lary rays, c, Cambium, /c, Fibres of Endophloeum. v I, Laticiferous vessels, e c, Cellular envel- 

 ope, Mesophlffium. p, Corky envelope, Epiphlceum. e p. Epidermis. 



Fig. 97. Vertical section of the same stem more highly magnified. /, Tracheze or spiral 

 fl,fl,fl, Woody fibres. The other letters as in fig. 96. 



