TRACIIEIDS 115 



The "camhial zone" occurs directly between the secondary 

 xylem and the secondary phloem and consists of the uniseriate 

 eamhium itself and a varying- number of layers of "maturing" 

 vascular elements. A careful study of the "cambial zone" will 

 lielp in understanding the direction of formation of secondary 

 xylem and secondary phloem and will furthermore shed some 

 light on the nature of the early developmental stages of the cells 

 making up these tissues. 



Within the ' ' cambial zone ' ' occurs the cylinder of secondary 

 xylem which is composed of two or three more or less concentric 

 layers or cylinders of xylem tissue, each of which is known as an 

 annual ring. In all woody stems of north temperate plants, an 

 annual ring represents the amount of secondary xylem formed 

 during a single growing season. The presence of annual rings 

 in the secondary xylem of perennial w^oody plants seems to be de- 

 termined to some extent by seasonal conditions, since the xylem 

 tissue formed in the spring (the so-called ''spring wood") dif- 

 fers somewhat in respect to the size, type and arrangement of its 

 cells from that formed in the summer (the so-called "summer 

 ivood"). It is due to this structural difference between "spring" 

 and "summer" wood that an annual ring appears distinct. In 

 many cases, the cells formed during late summer tend to be some- 

 what smaller and thicker-walled than those arising in the spring; 

 often the difference is emphasized by the localization of the ma- 

 jority of the vessels in the spring wood. 



The secondary xylem of Tilia is, like that of many woody 

 dicotyledons, a very complex "tissue system," consisting of 

 the following tj^pes of cells : 



(a) Vessel elements, which are large prominent cells, rather 

 polj'gonal in transverse section, and possessing large empty 

 lumina. In especially thin regions of your section you may be 

 able to see small bordered pits in cross-sectional view. Careful 

 focusing should reveal the "compound middle lamella." 



(&) Tracheids, which are smaller in size than the vessels and 

 are often more or less rectangular in shape as seen in transverse 

 section. In contrast to the vessels, the tracheids are frequently 

 arranged in definite radial rows. 



