ANATOIVIY OF STEMS 



227 



Tracheids are found in wood of many but not all species of angiosperms. 

 They are typically more or less spindle-shaped cells with thick walls which 

 are almost always lignified. As viewed in cross section they are usually angular, 

 Mature tracheids contain no protoplasm and hence are non-living like the 

 xylem vessels. The largest tracheids are about 5 mm. in length, and about 30 /a 

 in diameter, although most of them are smaller. The walls of tracheids are 

 pitted like those of vessels. Tracheids are water-conducting cells but in most 

 angiosperms they are relatively of much less importance as channels through 

 which water moves than the vessels. 



Tracheids are developed from the cambium by a process essentially similar 

 to that by which vessel segments are formed, except that the size and shape of 



Fig. 61. Stages in the development of a vessel. (A) enlarged cambial cell, 

 (B) cell still further enlarged ^vith secondary wall developed and pits formed, (C) 

 cytoplasm now only adjacent to walls, nucleus against transverse wall, (D) cytoplasm 

 has disappeared, transverse walls disintegrating, (£) transverse walls have disap- 

 peared; transformed cell now part of vessel. Redrawn from Eames and MacDaniels 

 (1925). 



the resulting cells is different, and that there is no coalescence of the individual 

 elements such as occurs in the formation of vessels. 



The xylem of the angiosperms also contains fibers of various types, which 

 in general are similar to the tracheids in structure except that usually they 

 have thicker walls, smaller lumina, and fewer and smaller pits. Fiber cells 

 are non-living and their walls are lignified. Because of their thick walls and 

 small lumina the fibers do not play an important part in the movement of 

 water through stems. 



The xylem tissue of practically all angiosperms also contains wood par- 

 enchyma cells which, unlike the elements already described, remain alive for 

 some time after their differentiation. Usually death of the wood parenchyma 

 cells does not occur until the wood of which they are a part is converted into 

 heartwood. Wood parenchyma cells are generally somewhat elongate, and 

 roughly four-angled in cross section. They occur in the xylem as vertical 

 series of cells placed end to end. The strands of wood parenchyma thus 



