CELL-CONTENTS AND FORMS OF CELLS. 273 



the leaves, and from the leaves the products of photosynthesis, 

 as well as other plastic substances, are distributed through some 

 of the tissues of the bark to other parts of the plant. The tissues 

 or elements of the wood which conduct food materials are of sev- 

 eral forms and include tracheae or vessels (also called " ducts "), 

 tracheids, and conducting parjenchyma; and the elements of the 

 bark which transport the assimilable materials comprise the lep- 

 tome and conducting parenchyma (Fig. 141). Water-conducting 

 elements (TRACHEAL ELEMENTS) co-mprise the vessels (tracheae) 

 and the tracheids, which resemble each other, except that the latter 

 are single cells of prosenchymatic shape, while the former are 

 very long tubes, varying from cylindrical to prismatic in shape, and 

 consist of long rows of cells which are superimposed length- 

 wise, the transverse walls being usually obliterated. 



' 



FIG. 143. Forms of tracheae or vessels. A. Longitudinal section of stem of Cucurbita 

 Pepo showing various forms of tracheae: A, annular; S, spiral; D, double spiral; C, close 

 annular; R, reticulate. B. Tracheae in glycyrrhiza rhizome: W, wall; B, bordered pores; 

 P, oblique simple pores. 



The tracheae or vessels are formed by the disintegration and 

 removal of the transverse walls between certain superimposed 

 cells, forming an elongated cell or tube, which occasionally retains 

 some of the transverse walls (Figs. 142144). The longitudinal 

 walls are relatively thin and consist of lignocellulose, giving pro- 

 nounced reactions with phloroglucin or aniline sulphate. 



Four types of vessels or tracheae are known : annular, spiral, 

 reticulate, and porous. Those having the thickenings in the form 

 of horizontal or oblique rings are known as ANNULAR TRACHEA; 

 those having the thickenings in the form of spirals, which usually 

 run from right to left, are known as SPIRAL TRACHEAE; those 

 having the thickenings in the form of a reticulation are known as 

 18 



