94 



VEGETABLE HISTOLOGY. 



In perennial dicotyl stems the cambium zone forms yearly a 

 new layer of woody tissue on its inner edge, thus increasing 

 the diameter of the plant. Moreover, the beginning and end of 

 the year's growth usually differ in appearance, and thus the 

 "rings of growth" are distinguished, and by their number the 

 age of the plant also. Monocotyl steins have no cambium in 

 the bundles after they are mature, hence such stems soon cease 

 to increase in diameter. They are, as a rule, slender stems. 



Study the whole cross-section as a type of the more woody 

 dicotyledonous stems and contrast it with the stem of the 

 Spiderwort studied above. It is the definite arrangement of the 

 bundles in a single circle, with xylem ends centrally and phloem 

 ends exteriorly, that gives rise to a true bark and a central 

 woody cylinder in dicotyls. 



Make cross-sections of stem of Lizard's Tail as a type of herba- 

 ceous dicotyledonous plants. 



Stain with phloroglucin. The bundles are arranged in a 

 circle, but are not grown together. They are separated by soft 

 parenchyma tissue, which forms 

 the greater portion of the sec- 

 tion. There is a cambium zone 

 between phloem and xylem. In 

 the xylem, next the cambium, 

 are some large ducts with a few 

 parenchyma cells mixed in. 

 Next to these are some ducts 

 of smaller diameter and then a 

 semicircular zone of thick-wall- 

 ed wood fibres marking the 

 limits of the xylem of the bun- 

 dle. In the phloem, next to the 

 cambium, is a mass of soft tis- 

 sue composed of sieve cells, par- 

 enchyma cells and a few secre- 

 tion cells. The phloem is 

 bounded by a layer of thick- Fig. 

 walled bast fibres with pore- 

 canals. This layer passes 

 around to meet the layer of 

 wood fibres of the xylem, forming thus a sort of sheath to the 

 bundle. 



Note that the arrangement of the tissues in this stem is the 

 same as that in the Bittersweet, the difference being one of de- 

 gree rather than kind. There is far less wood in the section. 

 The type of bundles and the arrangement in a single circle is 

 the same. 



Sketch the whole cross-section of Bittersweet and Lizard's 

 Tail and make a more detailed drawing of a segment of each 

 section. 



57. Cross-section of a bundle of 

 a, parenchyma ; b, air 

 space ; c. large duct ; d, bast fibres ; 

 h, wood fibres ; e, soft bast ; f , cam- 

 bium ; g, smaller duct (Bastin). 



