152 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 



Fig. 106. — The sieve plate, on 

 surface and in section, of a typical 

 sieve-tube of Squash ; X 400. 

 Note that the protoplasm extends 

 without break through the open- 

 ings in the plate. (From Stras- 

 burger.) 



the translocation occurs along 

 special cells and vessels of 

 the veins. Thus the carbo- 

 hydrates, as already explained 

 (page 31), move along the vein 

 sheath in the leaves, and along 

 parenchyma cells, chiefly of 

 the phloem but partly of the 

 xylem, in older bundles. All 

 such conducting cells are elon- 

 gated, possess plain walls, and 

 contain protoplasm. The pro- 

 teins have their clear paths of 

 conduction in the sieve-tubes, 

 of which this is the distinctive 

 function. The sieve-tubes are 

 elongated, thin, and smooth-walled cells, arranged in lines, 

 with their intermediate walls perforated (Fig. 106) ; and they 

 have always a lining of living pro- 

 toplasm. Associated with them are 

 certain companion cells, which have 

 seemingly a part, though an un- 

 known one, in their function. So 

 far as known, the proteins move 

 along the sieve-tubes solely by dif- 

 fusion from the places of greater to 

 places of lesser abundance. The 

 extreme slowness of this method, 

 however, suggests that the living 

 protoplasmic linings of the sieve- 

 tubes may act in some way to force 

 the movement, though there is no 

 evidence thereof. 



Thus it is true in general that Fig. 107. — The result of 



,i » . . constriction by a label wire 



the movement of water in stems is on a growing shrub . (From 

 chiefly in the wood, and the move- Bailey.) 



