72 THEMICROSCOPIST. 



covered by deposits, giving rise to porous cells (>, Fig. 24), or 

 dotted cells (a, Fig. 24); at other times the thickening matter 

 is in the form of a ring or spiral coil, constituting annular (c, 

 Fig. 24) and spiral cells (d, Fig. 24). 



Vascular Tissue, prepared by maceration and dissection, 

 presents many interesting subjects. Spiral vessels, c, Fig. 23, 

 consist of membranous tubes with conical extremities, inter- 

 nally furnished with one or more spiral fibres. As the vessels 

 grow, the spiral fibre breaks into short pieces, forming rings. 

 The vessels are then called annular, Z>, Fig. 23. If the pieces 

 of fibre are still shorter, they are called dotted or reticulated 

 vessels, c?, Fig. 23. The root of the garden rhubarb, the 

 stem of the hyacinth, the leek, &c., furnish examples. 



A peculiar form of vessel is met with in the common carrot ; 

 it is obtained from a root in a layer between the yellow central 

 portion ancl the red annulus. 



Sections of Wood. These are cut thin, so as to allow them 

 to be viewed as transparent objects. Hard woods, containing 

 gum, resin, &c., should be soaked in essential oil, alcohol, 

 ether, &c., before mounting. By transverse slices, a variety 

 of beautiful lace-like objects may be obtained, but little infor- 

 mation is acquired from them of the real structure of the wood. 

 For this purpose, if the tree is of the endogenous and branch- 

 less kind which grow by additions to the interior a vertical 

 section is also necessary. If the tree be an exogen, two verti- 

 cal sections will be required in addition to a transverse one. 

 The exogens grow by annual layers exteriorly under the bark, 

 and are branched. In these one of the vertical sections should 

 be radial and the other tangental. The radial vertical section 

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

 the small portions of pith which proceed horizontally from the 

 centre, enclosed in a sheath of woody fibres. The frequency 

 and size of the medullary rays determine the number and 



