18 THE MICROSCOPE, Feb. 



The stem of a plant when seen under the microscope 

 generally consists of three parts — pith, wood, and bark. 

 The pith is in the center (P), the bark outside (B), and the 

 w^ood (W), between the pith and the bark. This wood is 

 separated into segments in the exogenous plants, so-called 

 because growing from the outside, by medullary rays 

 (M. r.) resembling the spokes of a wheel. They are com- 

 posed of unchanged cellular tissue that extends from the 

 pith to the bark. The pith (P), is composed of cellular 

 tissue, and the cells are generally hexagonal in shape 

 when seen transversly as in the cut. vSometimes they 

 present a ragged appearance from being torn by the 

 rapid growth of the plant and in the Monocotyledons the 

 cellular mass in which are the fibro-vascular bundles cor- 

 responds to the pith and medullary rays together. Some- 

 times in the case of hollow stems the cells composing the 

 pith have cracked and dried away or are attached to the 

 sides in ragged patches. Immediately surrounding the 

 pith is the medullary sheath (M. s.) composed of a deli- 

 cate membrane, usually spiral in structure; this forms the 

 limit of the woody portion of the stem. The woody por- 

 tion extends from the medullary sheath to the cambium 

 layer (C.I.). The latter is situated between the wood and 

 the bark. The cambium is made up of a single ring of 

 delicate cells which changes into cellular and vascular 

 tissue thus forming new lateral cells and occasionally a 

 new radial cell. When continued, these form a new ring 

 of wood and also a new layer of bark. At the same time 

 they piece out and extend the medullary rays as fast as 

 the stem expands. 



X. — SECTION OF STEM OF IRONAVOOD. 



The pith (P), medullary sheath (M. s.), medullary rays 

 (M. r.), Wood (W), Cambium layer (C. 1.) and bark (B) 

 are all seen in the Ironwood as they were in the beach, 

 but with marked differences. The pith is now less con- 



