WATER TISSUE 275 



Each sieve tube cell has one or more companion cells 

 alongside of it (Fig. 43, C). These are typical " protein 

 cells " (see above). In most Seed Plants they are cut 

 off by division from the mother cell of each sieve tube 

 segment at an early stage of its development, before 

 the sieve tube cell acquires its special character. 



The sieve tubes are certainly the main conducting 

 channels of organic nitrogenous substances (amino-acid 

 compounds, amides and perhaps the simpler proteins) 

 and of sugars, but we know very little indeed of the 

 manner in which the transport takes place. 



Living Cells poor in Protoplasm. Water Tissue. — At 

 the other extreme from secretory and protein cells are 

 water cells, which have a layer of cytoplasm lining the 

 wall very thin in comparison with the size of the cell 

 (Fig. 43, E, F), the vacuole being filled with a very 

 watery sap, i.e. an extremely dilute solution. These 

 cells are, however, only an extreme form of the typical 

 thin-walled parenchymatous cell. The bodies of 

 succulent plants consist mainly of such thin-walled 

 water cells, but in the typical succulents (Cacti) the 

 cells contain a large quantity of pentosans (substances 

 having a similar relation to pentose sugars that the 

 celluloses have to the hexose sugars) which attract 

 water strongly. The active cells, for instance the 

 photosynthetic cells near the surface of the succulent 

 stem or leaf, can draw upon the store of water in the 

 water tissue. 



The epidermal cells act as water cells in an ordinary 

 leaf and provide a small supply of water which is drawn 

 upon by the mesophyll cells if they are losing water 

 to the air more quickly than they can be supplied 

 from the water-conducting system. In some cases 

 isolated epidermal cells are many times the size of 



