58 STRUCTURAL BOTANY 



tubes when mature have no nuclei. Each of their 

 constituent cells has a nucleus to begin with, but 

 during the development of the sieve-tube it breaks 

 up into fragments, and ultimately disappears alto- 

 gether. 



Where sieve-tubes are in contact with each other 

 side by side, their lateral walls are also perforated, 

 not all over, but in certain parts. Through the per- 

 forations, whether of the transverse or of the longi- 

 tudinal walls, the contents of the sieve -tube are 

 continuous. We must remember that this perforation 

 of the cell-walls and open communication between the 

 protoplasm and neighbouring cells is not peculiar to 

 sieve-tubes. We have already learnt that living cells 

 generally have their protoplasm continuous through 

 minute perforations in their walls. The perforations 

 in the sieve-tubes, however, are often much larger 

 than those of ordinary cells, and continuity of the 

 protoplasm was recognised in them long before it was 

 even suspected to exist between other living cells. 

 That part of the cell-wall of the sieve-tube which is 

 perforated is called the sieve-plate. 



2. The Companion-Cells 



The sieve-tubes are accompanied by narrow, longish 

 cells, each of which is densely filled with protoplasm, 

 and contains a large nucleus, which is permanently 

 retained. These cells are called the companion-cells. 

 In a transverse section the companion-cells look as if 

 they had been cut off from the corners of the sieve- 

 tubes, and that is how they really originate, for 



