188 PHYTOTOMY. 



shews, that its apparent fibres are really, tubes, entirely of 

 the same construction with tlie sap-vessels which have been 

 already described. The bundles of these tubes ai'c bent 

 from one another, where the horizontal and radiated cells of 

 the bark intersect them. It is hence that the appearance of 

 meshes is formed ; (Tab. V. Fig. 3.) No trace of spiral ves- 

 sels is found in the inner bark. 



296. 

 This is peculiarly that part of the stem in which the juices 

 ascend, as may be distinctly seen by a horizontal cut into the 

 trunk during spring. The higher these juices ascend, the 

 more are they changed into the organic slime, which is de- 

 nominated the Generative Sap , and, in this sense, the inner 

 bark may be considered as the organ from w hich all the other 

 parts are produced. 



297. 

 The peculiar woody circle, which lies beneath the inner bark, 



is composed of all the three primitive forms. In its earliest 

 state, when it most resembles pith, it consists only of the pri- 

 mitive spiral vessels, together wdth the sap-vessels which con- 

 stantly accompany them, and which are intersected by the ra- 

 diated and converging rind-cells, sent towards them through 

 the inner bark. It is evident that these latter vessels must al- 

 ways become closer set, the nearer they approach the pith. The 

 more recently deposited layers contain for the most part vasa 

 scalaria and punctured vessels, but more seldom a primitive 

 spiral vessel between the individual layers. 



298. 

 In the older branches and stems of many trees, the distinction 

 between the younger and older woody layers, is easily ob- 

 served. The former, which are usually called the alburnum 

 {alburnum)^ are known by their white colour, spongy texture, 

 and inferior durability. Many trees, which either grow ra- 

 pidly, or whose organisation is peculiar, deposit nothing but 

 alburnum ; in many trees, what is called debility of the al- 



