THE STEITCTUEE OF THE STEW 



361 



ment, whether from cambium or procambium. The segment of 

 the merismatic cell which is about to give rise to a sieve tube 

 cuts off by longitudinal walls one or sometimes two segments, 

 which are much smaller than the remaining one. These con- 

 stitute the companion cells (fig. 744, c) ; each remains filled with 

 protoplasm and contains a large nucleus. The remaining cell 

 slightly thickens its end walls, which become perforated, forming 

 sieve plates, through which the contents of the adjacent cells com- 

 municate (fig. 744, s.p.). The nucleus breaks up and disappears, 

 so that the mature sieve tube has not a nucleus. The sieve 

 plates have later a deposit of callus vipon them. 



FiCt. 744. 



Fifj, 744. Sieve tube and companion cells. 

 s.p. Sieve plate, c. Companion cells. 



.t. Segment of the sieve tube. 

 : 540. After Strasbm-ger. 



In the Gymnosperms the sieve tubes have no companion colls. 



The cambium cells, though usually elongated prisms in shape, 

 are not always so. Those cells which continue tlie primary me- 

 dullary rays (fig. 74:2, if.c), or which form secondary ones, divide 

 transversely, becoming almost cubical. The segments which they 

 contribute to the medullary rays have thus always this shape. 



By this mode of behaviour of the cambium ring, wood is 

 continually formed centrifugaUy, and the stem increases in thick- 

 ness. The activity of the cambium continues for the greater 

 portion of each year, only ceasing during the winter, and conse- 

 quently a new zone of wood is produced annually. The vessels 



