BOTANY OF THE LIVING PLANT 



carbohydrates are branslocated, while nitrogenous compounds may 

 mn \ imino-acids, or as amides. 



I, [s now held by most authorities that longitudinal translocation 

 metabolic materials in the plant is chiefly through the sieve-tubes 

 the phloem, excepl for the upward passage of inorganic salts, 



which in all probability occurs with the 

 T< £1 transpiration stream in the xylem (see 



J I Chapter VII.). The structure of the 



V f sieve-tubes (see p. 48) appears to facili- 



tate a lengthwise flow of materials 

 through them, inasmuch as they are 

 elongated and the cross-walls are per- 

 forated to form the sieve-plates, per- 

 mitting of free movement of material 

 from one tube to the next. Further, 

 the contents of sieve-tubes are rich in 

 carbohydrates and nitrogenous sub- 

 stances. More direct evidence is fur- 

 nished by ringing experiments. It can 

 be shown that the removal over a short 

 zone of a leafy stem of the tissues 

 FlG . 86. external to the cambium interferes with 



Lower parts of willow cuttings which th downward translocation of materials 



had been induced to form adventitious 



mots by standing in water. The right- r ^ J eaves . as indicated by re- 



hand cutting was initially ringed ' ' 



■hotly above the base. Note the effect duced growth of the parts of the plant 

 .•11 the position of root-production. ( x §.) » r r 



below the ring, and by accumulation of 

 < arbohydrates and nitrogenous compounds above the ring. A tree is 

 Usually killed if its trunk is ringed, because the ring interrupts the 

 flow of food substances from the leaves to the roots, which are in 

 consequence gradually starved. The result of ringing cuttings is 

 illustrated in Fig. 86. The production of the roots above the ring 

 it least partly due to the arrest of the flow of nutritive materials 

 which diffuse from the upper part of the cutting to the region of 

 root-initiation. Corresponding results have been obtained in connec- 

 tion with the upward movement of organic materials through the stem, 

 such as occurs in a deciduous tree in spring, w r hen reserves stored in 

 the trunk are transported up to the opening buds. These ringing 

 experiments indicate that the conveyance of organic materials is 

 effected not in the xylem but in some more external tissue : 

 and the considerations already mentioned point to the conclusion 

 that the phloem is the tissue in question. This view has received 



