SUNFLOWER. STEM. 61 



(a.) Pitted vessels, which are the largest, having 

 very large cavity, walls 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. 



(y.) 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 

 disorganised. 



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. 



c. Parenchyma, which is to be found more 

 especially around the vessels near the central limit of 

 the bundle. The phenomenon 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 (reactions), and 

 retain their protoplasmic contents. 



The cells termed thyloses (Tullen) are properly included under 

 the term xylem parenchyma, being derived directly from this 



