322 



MANUAL OF BOTANY 



the two segments communicate. The protoplasm of the sieve 

 tube is considerably changed, being, with the exception of a lining 

 layer, transformed into a slimy material which is especially con- 

 spicuous in the neighbourhood of the sieve plate. The adult 

 tube contains no nucleus. 



The companion cells are so called because they are out off 

 from the sieve-tube segments at the time of their first appearance. 

 They are smaller in diameter than the tube itself, and contain 

 protoplasm and nuclei (,/?</. 695, c). 



In the Angiosperms the sieve plates at the end of summer 

 become blocked up by an addition to the callus, and remain 



Fre. 69fi. 



Fig. G9G. Sieve tubes of lihamnus purshiana. A, B. Middle portion oi! a tube, 

 showing section of sieve plates, s.p. c. Callus deposit on a sieve plate. 

 c. Termination of a sieve tube, with sieve plates, n.p., on the lateral 

 walls. X 320. 



closed during the winter, communication being re-established in 

 the spring. Sieve plates often occur upon the lateral as well as 

 the end walls of the segments (fig. 696). In many Ferns they 

 are confined to the former position, and are very numerous in 

 each tube. 



In all plants lower in the scale than the Angiosperms, sieve 

 tissvie contains no companion cells. 



Tissue much like sieve tissue occurs in certain regions of 

 the thallus of one of the brown seaweeds (Macrocystis). In 

 some others of the Laminarias similar structures are found, 

 which are called frumpcf hyphcr. It is, however, doubtful if 

 their segments communicate, 



