44 PRACTICAL BOTANY 



absorption of the transverse or oblique septa. Note instances 

 of this partial or complete absorption. According to the various 

 markings, or thickenings of their walls, the vessels may be 

 grouped under the following heads, the first named being the 

 nearest to the periphery of the stem : 



a. Pitted vessels, which are the largest, having very wide 

 cavity : their walls are marked with pits which appear oval in 

 surface view, and which have the same characters as the round 

 bordered pits of Pinus (see p. 138). 



Having observed the pits in surface view, focus so as to 

 obtain a longitudinal optical section of one of the walls ; or 

 better, find a place where the preparation is so thin as to show 

 this in real section. Compare this with what was seen in 

 surface view. 



#. Spiral vessels found in the part of the xylem nearer the 

 pith, those nearest the pith having the spirals less closely coiled. 

 Note transitional forms (irregularly reticulated) between spiral 

 and pitted vessels. 



y. Annular vessels found at the part of the xylem directly 

 adjoining the pith : the thickening is here in the form of rings; 

 in mature stems these vessels are usually more or less dis- 

 organized. 



b. Fibrous cells (wood-fibres), which are long and pointed : 

 it is difficult to follow one individual fibre throughout its whole 

 length, owing to its taking a sinuous course, the fibres being 

 interwoven one with another : their walls are lignified and 

 pitted : the cell-contents are reduced or absent. 



c. Parenchyma, which is to be found more especially around 

 the vessels near the central limit of the bundle. The phe- 

 nomenon of thyloses is the result of the encroachment of these 

 cells on the cavity of the vessels. The normal individual cells 

 are oblong with square ends, they have cellulose walls, and 

 retain their protoplasmic contents. 



5. The central pith is composed of parenchymatous cells, 

 with thin walls consisting of cellulose : the walls are slightly 

 pitted : these cells have lost their protoplasmic contents in 

 many cases, and especially near the centre of the stem. 

 Occasional resin passages niay be foun4 in the pith, 



