THE STEM. lOI 



The really important part is not the pith, which is 

 of no special value, but the cambium layer. The cells 

 here have a very remarkable shape, which governs all the 

 cells that are formed from them. The shape is best 

 understood by making a model as follows : Take two 

 pieces of cardboard ( 1 8 inches long by 3 broad), lay 

 one upon the other, and attach the two short ends by a 

 strip of gummed paper. Then bend these cardboards 

 slightly so as to leave an interval between them. 

 Such a shape has many remarkable possibilities. Thus 

 the cambium cells, when they divide, simply split 

 lengthwise into two exactly similar shapes. Those 

 cut off upon the inside become part of the xylein, those 

 on the outside turn into phloem^ whilst those formed in 

 the medullary rays turn into new medullary ray-cells. 



The xyleni contains the following important modified 

 cells. Water-conducting cells, tracheids, which are 

 of the same shape as the cambium cells, but their 

 living matter has been used up in forming a thick 

 woody covering inside the original cell wall. The 

 wood is not a continuous deposit on the inside but is 

 laid down in the form of rings, spirals, or a network of 

 woody matter." Thus, much of the original unthickened 

 wall is left between the woody deposits, and the tracheid 

 is strong enough to assist in supporting the other 

 tissues, and yet forms a conduit through which the 

 water ascends. In Pine timber, there is a nearly con- 

 tinuous woody deposit, but with pores of a peculiar 

 shape, which probably act as valves allowing any 

 injured tracheids to be cut off from the others. In 

 such trees as the Lime, Ash, and Horse-Chestnut, 

 vessels are found, composed of tracheids arranged one 

 above the other, and with the divisions absorbed, so 

 that the line of tracheids is an open pipe up which the 

 (Br) L 



