52 



EXOGENOUS OB UICOTYLEDONOtTS STEM. 



quadrangular cpUs, a d, form the epidermis, to which succeeds the 

 cellular tissue of the bark, b. The latter surrounds a bundle of bast 

 (phloem) fibres, c, and ligneous layers of inner bark, with laticiferous 

 vessels, d d', which are separated, in the direction towards the interior, 

 by a layer of cambium cells, d", from the proper vascular tissue (xylem), 

 consisting of pitted vessels with thick walls, g g, and others with thin 

 walls, h h, mixed with woody tubes, e. 



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 



Fig. 116 St*. 



Fig. 116 ter. 



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

 rays being at the same time continued from within outwards. This is 

 represented in fig. 116, where 1, 1 indicates the section of the stem 

 of the first year's growth (the letters referring to the same parts as in 

 figs. 114, 115); and 2 shows the interposed zones of the second 

 year, by which the diameter of the stem is increased. 



The Pith, or the central part of a dicotyledonous stem, is com- 



Fig. 116. Vertical Bection of a brancli of common maple (^Acer campestre) two years old, 

 where (1, 1) indicates the portion formed the first year, and (2) that formed the second. 

 The letters as in figs. 114 and 115. Fig. 116 &w. Certain parts of the preceding magnified, in 

 order to show the structure of the vessels and cells, as well as their form and direction.- 

 Fig. 116 ter. A portion of a pitted vessel from the gourd, magnified. 



