88 MICROSCOPIC OBJECTS. 



real structure of the wood. In order to understand the 

 form and arrangement of the various parts, one vertical 

 section is also absolutely necessary, if the tree is of the 

 endogenous and branchless kind — ^which grow by addi- 

 tions to the interior ; while if an exogen two vertical 

 sections wiW. be required in addition to the transverse one. 

 The Exogens grow by annual layers exteriorly under the 

 bark, and are branched. In the endogens it is unimpor- 

 tant in what direction the plane of the vertical section is 

 made, providing it be parallel with the axis ; but in the 

 exoffens one of the vertical sections should be radial and 

 the other tangentah The radial vertical section exhibits 

 the number and size of the medullary rays ; that is, the 

 smaU portions of pith which proceed horizontally from 

 the centre, inclosed in a sheath of woody fibres. The 

 frequency and size of the medullary rays determine the 

 number and strength of the branches of the tree. This 

 section also exhibits in coniferous trees the beautiful disc- 

 like glands which adhere to the woody fibres. Tliese are 

 beautiful objects, and sometimes require a power of 200 

 to 300 diameters. 



The tangental vertical section is a slice across the me- 

 dullary rays ; it exhibits the form and arrangement of 

 the cellular tissue within them. All the vertical sections 

 show the form, size and connection of the woody fibres. 

 Spiral, reticulated and dotted vessels, &c., and are by far 

 more instructive than the transverse ones. Of course in 

 some kinds the vascular structure is not developed so 

 much as in others : thus there may be an absence of 



