274 TISSUE ELEMENTS OF SEED PLANTS 



the secretory cell type, is the protein cell (Fig. 43, C), 

 which is constantly associated with the cells (sieve tubes) 

 that conduct organic food substances. The exact 

 functions of these protein cells are not understood : 

 perhaps they secrete enzymes which act on the substances 

 passing along the sieve tubes. Protein cells have 

 large conspicuous nuclei, and are very rich in protein 

 contents. 



Sieve Tubes are the main channels of transport of 

 organic substances from one part of the plant to another. 

 Each sieve tube consists of a row or chain of elongated 

 cells (Fig. 43, C), comparable with the medullary cell 

 chains of Fucus. The cross walls separating the 

 successive cells of the chain early become perforated by 

 holes eaten out as it were by the action of an enzyme 

 'Which in extreme cases may be as much as 5 //, in 

 diameter, and through which pass relatively coarse 

 "slime strands" connecting each cell with the next 

 cell of the tube. The perforated cross wall is called 

 a sieve plate (Fig. 43, D). The vacuoles of the sieve 

 tube cells become filled with a highly nitrogenous 

 thin mucilage. The layer of cytoplasm lining the walls 

 remains, but the nucleus disappears, and the cells can 

 hardly be considered " living " in the full sense. On 

 the sieve plate there is eventually formed a mass of 

 a carbohydrate substance called callose which blocks 

 the holes in the plate (Fig. 43, C). Callose refracts 

 light strongly and takes a deep colour with certain 

 stains. The blocking of the holes by the callose 

 apparently interrupts the transport of substances 

 along the tube ; later the callose is dissolved, and 

 ultimately the sieve tube becomes empty and no longer 

 functional, for instance in the parts of woody plants 

 which are several years old. 



