CELLS AND TISSUES 9 



to form a torus. The bordered pit is found chiefly in the walls of 

 the vascular elements of secondary xylem; and, where these abut 

 parenchymatous cells, the pit may be bordered in the wall of the 

 vascular element and simple in the wall of the parenchymatous 

 cell, thus forming a half-bordered pit. Other types of secondary 

 thickening found in vascular elements, including the annular, 

 spiral, scalariform, and reticulate types, are described under xylem 

 tissue. 



Structure of the Cell Wall. — The detailed structure of the 

 cell wall has been investigated by a number of methods, including 

 X-ray analysis, differential staining, chemical and ash analyses, 

 polarized light, microscopic observation of both treated and 

 untreated materials, and hydration and swelling of the wall with 

 various reagents. As a result, several concepts of the wall struc- 

 ture have been proposed, which are here summarized. 



The oldest idea, and one which still receives widespread sup- 

 port, is the hypothesis proposed by Nageli (30). According to 

 this concept, the cellulose portion of the wall consists of elongated, 

 crystalline, submicroscopic units known as micelles., which are sepa- 

 rated from one another by colloidal materials. On the basis of 

 X-ray analysis, it is thought that the micelle consists of cellulose 

 groups made up of parallel chains of glucose residues to which vary- 

 ing lengths and numbers of glucose units have been assigned by dif- 

 ferent investigators. 



Sponsler (41, 41) has proposed a second idea of the structure 

 of the wall in which the cellulose units form a three-dimensional 

 space lattice; and has demonstrated that the molecules are arranged 

 in parallel layers, some of which extend lengthwise of the fibers 

 while others are at right angles or otherwise oriented at definitely 

 determined angles. The X-ray patterns and other data indicate 

 "that the units are attached in chains of indefinite length"; and 

 the lattice concept of wall structure does not necessitate the assump- 

 tion of "the existence of micellae as structural units of the cell wall, 

 that is, large units made up of many molecules, such as Nageli 

 proposed." 



A third theory presupposes the existence of larger, visible 

 cellulose units, which have been called by various names: fibrils, 

 cellulose units, ellipsoid bodies, dermatosomes, etc. This idea 

 has been supported by Liidtke (2.8), who described the units as 

 being enclosed by thin films of non-cellulose cementing material 



