14 



STEM. 



say that no diminution of it takes place by the pressure 

 of parts around it. 



73. This central system of an Exogenous stem is com- 

 posed alone of cellular tissue those cases in which 

 other tissues have been noticed in the pith, will be 

 mentioned hereafter. 



74. In some plants the pith disappears as the vege- 

 table grows, the stem of which becomes quite hol- 

 low; this arises from the rapid growth of the sur- 

 rounding structures, when young, being too great for the 

 pith to keep up with ; the result of which is, the pith is 

 torn to pieces. This is seen in the common Hemlock. 



75. The medullary sheath is a thin layer of a green 

 colour closely surrounding the medulla or pith. This 

 sheath is composed chiefly of spiral vessels, and is the 

 first formed of the vascular system of a plant. 



76. The woody layers are composed of vascular and 

 fibrous tissue, and exist in the shape of concentric zones 

 between the medullary sheath and inner layer of the bark. 



77. These zones of woody matter vary in number 

 according to the age of the stem ; as a general rule for 

 our climate, we may say that there is a zone of woody 

 matter for every year the plant is old. 



78. The innermost zones of woody matter are termed 

 the duramen, or heart wood, and outermost or last 

 formed zones, alburnum. 



79. The duramen is harder than other parts of the 

 wood, and often contains a great deal of colouring 

 matter, whilst the alburnum is younger and consequently 

 softer, and of a lighter colour. 



80. The medullary rays are laminae of cellular tissue 

 which traverse the woody layers from the centre to the 

 circumference, being connected in the centre with the 

 tissue of the medulla, and at the circumference with 

 that of the bark. 



81. These rays do not always run unbroken and con- 

 tinuous, but become incomplete as in the Conifer*. 



82. The bark lies external to the woody layers, and 

 is composed originally of four layers. 



