CH. vin THE HISTOLOGY OF THE TISSUES 



93 



20. Pitted, spiral, annular, 

 and reticulate vessels. 



If the walls are pitted (p. 78) they are called pitted- vessels. 

 When the thickenings of the walls appear to form a spiral, the 

 vessels are spoken of as spiral 

 (Fig. 120). When the markings 

 give to the vessel a reticulate or 

 netted appearance, such vessels are 

 termed reticulate (Fig. 120). If 

 the transverse walls have been per- 

 forated by a single round opening 

 while the rest of the walls remain 

 to form thick rings the vessels are 

 called annular. Vessels of the 

 above kinds are found in the wood p, G> 

 of all plants. The walls of all such 

 vessels are lignified. 



Sieve Tubes. In the formation of the sieve vessels or 

 sieve tubes the transverse walls are not completely broken 

 down, but they are perforated by fine canals through which the 

 protoplasm passes for such vessels keep their living contents. 



The wall which contains 

 the perforations is called 

 a sieve plate. In some 

 plants the longitudinal 

 walls may become simi- 

 larly perforated so that 

 sieve plates are also 

 formed there. The walls 

 of sieve tubes are always 

 unlignified, and the ves- 

 sels contain a watery cell- 

 sap. In close contact 

 with the sieve tubes, and 

 formed from the same 

 cells during development, 

 long narrow cells are 



formed, and these are called companion cells. The nuclei 

 of the sieve tubes are broken up and disappear, but the com- 

 panion cells keep both their protoplasm and nuclei. 



Kinds of Tissues. When a number of cells are intimately 

 connected, and perform the same kinds of work, they are 



FIG. 121. Parts of sieve tubes from Vegetable 

 Marrow. A, surface view of sieve plate ; 

 B, C, longitudinal sections, showing sieve 

 plates ; D, contents of sieve tube ; S, com- 

 panion cells ; PA, protoplasm ; C, lateral 

 sieve plate. (X 270.) (After Strasburger.) 



