EVIDENCE THAT THE PHLOEM CARRIES THE FOOD 159 



As was stated in Chapter II, all classes of plants do not pos- 

 sess the full complement of phloem elements here described. 

 In Gymnosperms and Vascular Cryptogams the companion 

 cells do not occur, and their place is taken by vertical rows of 

 parenchyma cells; and in Monocotyledons the parenchyma cells 

 are lacking. Neither are the sieve tubes alike in all respects 

 in the different classes and families of plants. In the Gymno- 

 sperms the primary end walls of the sieve tube members (cells 

 composing the tubes) are not dissolved away at the bottom of 

 the pits, as seems to be the rule in Angiosperms, and in the 

 latter class the size of the pores varies greatly in plants of dif- 

 ferent habits of growth. 



For the study of sieve tubes the squash and its allies, or the 

 grape, hop, or other climbing plant is chosen, because, in these 

 plants with slender stems, the sieve tubes and the pores through 

 the partition walls are found to be larger than in plants of dif- 

 ferent habit, evidently for the reason that, the stem being slender 

 and the crown of leaves relatively large, the tissues devoted to 

 food-transportation must be unusually efficient. In many 

 plants the pores in the sieve plates or partition walls are discerned 

 with the greatest difficulty because of their minuteness, and in 

 some cases they cannot be made out at all. 



Evidence that the Phloem Carries the Food. Micro- 

 chemical examination, in which chemical reagents are applied 

 to sections under the microscope, shows that the phloem is 

 filled with food substances. Chemical analysis of the expressed 

 contents of sieve tubes has found 7 to 10 per cent, of solid or 

 dissolved substances, of which 20 per cent, was proteid, 30 per 

 cent, amide (a nitrogenous food of simpler composition than 

 proteid), and 38 per cent, soluble carbohydrate. That these 

 substances are actually transported by the phloem is shown by 

 experiments in which the continuity of the phloem is wholly 

 or partially interrupted. When a ring of bark is removed down 

 to the phloem there is no significant disturbance in food trans- 

 port, as shown by the fact that growth in diameter continues below 

 the girdle as well as above, and the storage of food goes on in 



