74 PRACTICAL BOTANY 



during life water or gases. The cavity is continuous 

 owing to the partial or complete absorption of the trans- 

 verse 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. 



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. 



/3. Spiral vessels found in the more central part 

 of the xylem, those most central having the spirals more 

 closely coiled. Note transitional forms (irregularly 

 reticulated) between spiral and pitted vessels. 



7. Annular vessels found at the central limit of 

 the xylem, the thickening is here in the form of rings ; 

 in mature stems these vessels are usually more or less 

 disorganized. 



5. 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. 



