248 



GYMNOSPERMS 



The principal feature of the phloem is the sieve tube. While the 

 tubes are usually in rows, there is no breaking down of transverse 

 walls to form continuous tubes, like the xylem vessels of angio- 

 sperms. The transverse walls are as thick, or even thicker, than the 

 lateral, and just as permanent. The sieve tubes taper at both ends, 

 but not so sharply as the xylem tracheids, and often the ends of the 

 tubes are merely rounded. Their most characteristic feature is the 

 sieve area, or sieve plate (fig. 255), The plates 

 occur irregularly throughout the entire length 

 of the tube, but are confmed to the radial 

 walls. In a well-stained preparation the plates 

 look somewhat like nuclei, so that the sieve 

 tube looks like a multinucleate cell. 



JeffreYj^*" in his Anatomy of Woody Plants, 

 gives an excellent illustration of the phloem of 

 Pinus (fig. 256). In this figure the cells of the 

 phloem containing protoplasm^ — indicated by 

 the dotting — belong to one year's growth. 

 The older, dead cells, becoming crushed and 

 distorted, have lost all their contents. In the 

 living sieve cells the strands of the sieve 

 plates^looking like protoplasmic connections 

 — furnish an extensive communication be- 

 tween contiguous cells. After the first year, 

 as the cells die and lose their protoplasmic 

 contents, a callus forms over the sieve plates. 

 The callus is shown in black in the illustration. 

 Four large parenchyma cells are shown, two 

 in the living phloem and two in the dead. 

 They contain abundant protoplasm and some starch. Two types of 

 rays are shown. The cells of the ray at the left contain no starch, and 

 die with the phloem; while the cells of the one on the right contain 

 starch and are long-lived. 



In the course of evolution, the plates become more and more con- 

 fined to the ends of the cells, and finally, in their highest develop- 

 ment in some of the angiosperms, the tapering character is entirely 

 lost, the end wall is transverse, and the sieve character is confined to 

 the end wall. 



Fig. 255. — Pinus: 

 longitudinal section of 

 cellsof the phloem, show- 

 ing numerous sieve areas, 

 or sieve plates; X433. 



