II 4 THE ANATOMY OF WOODY PLANTS 



appearing black in the figure, which are present exclusively on the 

 radial walls of the elements, although in some instances the col- 

 lapsed state of the tubes gives rise to the appearance of a tangential 

 position for the bodies under discussion. The plugs mentioned 

 above constitute the callus, a substance which in the conifers 

 and the great majority of seed plants higher in the scale makes 

 its appearance on the sieve plate at the time the tube is in the 

 initial stages of collapse. The callus completely blocks the pores 

 of the sieve plate and, after persisting for a short time, disappears, 

 leaving the pores of the sieve area quite open. It is also apparent 

 that in the case of the ray cells on the left of the figure the callus 

 present is unilateral and occurs only on the side of the sieve tube. 

 Obviously the type of ray element appearing on the left in the 

 illustration is definitely related in its duration to the life of 

 the sieve tube, and both structures cease to be functional at the 

 same time. 



In the next figure (Fig. 88) the radial aspect of the phloem is 

 shown. Here the ray naturally appears in longitudinal section. 

 Above and below in the region of the phloem it shows vertically 

 placed cells which contrast in the position of their axis of greatest 

 length to the central elements of the radial parenchyma. These so- 

 called "erect cells" correspond to the elements shown in the ray 

 to the left of Fig. 87 and are similarly characterized by the absence 

 of the starch, which is a feature of the contents of the central or 

 prone cells of the ray. In passing toward the right from the cam- 

 bium the protoplasmic filling of these elements becomes less 

 abundant, until finally, where the callus presents itself in face 

 view, it disappears altogether, in this respect presenting a con- 

 trast to the central cells which retain their protoplasm and starch 

 indefinitely. The sieve tubes now appear in longitudinal view, 

 and nearer the cambium they have a delicate protoplasmic lining 

 often containing some small grains of so-called transitory starch. 

 The sieve areas, or sieve plates, are presented to the observer in 

 face view, and toward the extreme right callus plugs occlude their 

 pores and at the same time the tube has lost its living contents. 

 .It is clear in the radial view, as in the transverse, that sieve tube 

 and marginal erect cell cease to exist and develop callus plugs 



