EXOGENOUS OE DICOTYLEDONOUS STEM. 



51 



at least one-half of the whole diameter,its cells diminishing in size as they 

 approach 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 circumference, dividing the vascular circle 

 into numerous compact segments, which consist of woody vessels, / b, 

 and of pitted vessels, vp. These are surrounded by a moist layer of 

 greenish cellular tissue, c, called the cambium layer, which is covered 

 by three layers of bark, /c, e c, and p, with laticiferous vessels, v I,' the 

 , whole being enclosed by the epidermis, e p. On making a thia vertical 

 section of a portion of the same branch, and viewing it under the 

 microscope, the parts composing the different portions become more 

 obvious (fig. 114). The pith, m, with its hexagonal cells decreasing 





Fig. 114. 



Fig. 115. , 



in size outwards, surrounded by a narrow fibro-vascular zone, the 

 meduMry sheath, consisting chiefly of spiral vessels, t ; the medullary 

 ray, rm; the vascular zone, consisting of pitted vessels, v p, of large 

 diameter, and forming the large round apertures seen in a transverse 

 section ; the fibres of the wood, / I, with their thick walls and smaller 

 apertures ; the inner bark or liber, / 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 epidermis, e p, having hairs scattered over its surface. 

 A transverse section of a bundle of vascular tissue of a dicotyledonous 

 plant, magnified 230 times, is represented in fig. 115. The arrow 

 indicates the direction from within outwards. We here perceive the 

 vascular bundle surrounded by a laxge-celled tissue (a db f). The 



Fig. 114. Vertical section of the same stem more highly magnified, t. Tracheae or spiral 

 vessels. /!, /!j /i, Woody fibres. The other letters as in fig. 113. Fig. 115. Tra-nsveise 

 section of a bundle of vascular tissue of a Diootyledonons plant, a d, Epidermis, t. Large- 

 celled tissue of bark, e, Fibres of bast layer, d, d'. Woody layers and laticiferous vessels 

 of Inner bark, d". Cambium cells, g g, and h h, Large pitted vessels, i, Woody tubes. 

 /, Large cells. 



