28 Plant Physiology 



this : a tissue may be primarily important for a specific 

 type of action, but differentiation is not commonly so 

 complete as to render it unserviceable in many correlated 

 activities. 



21. Tracheae or vessels. - The vessels are formed 

 from rows of elongate cells by the absorption of the inter- 

 vening walls coincident with the disappearance of the 

 protoplasm. Such cell-cavities, or ducts, may extend 

 continuously for several centimeters in length, and they 

 are especially important in the conduction of water in 

 angiosperms generally. These ducts also show usually 

 the annular or spiral thickenings previously referred to. 

 Both this and the preceding type are commonly associated 

 with at least a small amount of parenchyma, and it is 

 probable that their physiological properties depend to 

 some extent upon the latter. 



22. Sieve tubes, - The sieve tubes are also elongate 

 cells, but they are peculiar in the fact that the protoplasm 

 in the adjacent members of the cell-row is continuous by 

 means of very distinct connecting pores through the 

 intervening walls. These walls are thickened and form 

 a so-called cell-plate, a perforate plate through which, 

 therefore, the protoplasm is continuous (Fig. 6). Cell- 

 plates may also occur at points of contact in more or less 

 vertical walls. 



Another peculiar feature of such sieve cells is the fact 

 that the nuclei become disorganized, but the cytoplasm 

 remains. These cells, however, are in close contact with 

 certain cells which are typical parenchyma elements, the 

 companion cells, the latter containing both cytoplasm 

 and nuclei. The sieve tubes usually occur in the woody 



