574 TRANSLOCA TION 



in which growth or metabolism is active. The vessels of the wood and 

 phloem are surrounded by tissue-elements which act as intermediaries 

 between the conducting channels and the external tissues, and the branch- 

 ing and intercommunicating system of vascular bundles provides for the 

 transport of water and food-materials to all parts of the plant. 



The division of labour is always relative in character and can never be 

 absolute, for every living cell and every tissue has necessarily the power of 

 absorbing and excreting nutritive substances and metabolic products along 

 with water, while the water-channels convey a certain amount of food- 

 material in the shape of the ash constituents drawn from the soil. Sugar 

 and other soluble nutrient substances derived from the reserves in the root, 

 stem, tubers, &c., may be conveyed in a similar manner if they pene- 

 trate the water-conducting tracheae, and in places where the sap is rich 

 in sugar, &c., as in the birch (Sect. 43), large quantities of plastic sub- 

 stances may be transferred by means of the wood-vessels. This is not, 

 however, possible in other cases, for both the sap exuded from a wound 

 and that obtained directly from the wood may contain hardly any organic 

 material whatever, as in the vine, &c. Similarly in herbaceous plants the 

 water-channels seem only to be utilized to a slight extent for the con- 

 veyance of organic food (Sect. 109), and moreover transference through the 

 xylem is practically possible only in one direction, for the ascending stream 

 of water carries the dissolved substances with it and almost entirely prevents 

 any downward diffusion. It was in order to secure the possibility of trans- 

 ference in all directions that a certain division of labour became necessary, 

 and a special conducting-tissue has been evolved for organic food-materials, 

 while, as might be expected, the conducting elements as well as the ground- 

 tissue of the phloem have acquired no special importance in the rapid 

 transference of water through the plant. 



Not only proteids but apparently all plastic substances are preferably 

 conducted through the phloem, for even when the bark of a tree is removed 

 down to the latter, carbohydrates are still transferred over as wide distances 

 as before l . During translocatory activity the living elements of the phloem, 

 and especially the sieve-tubes, contain an abundance of proteids, carbo- 

 hydrates, fats, amides and phosphates, and the results of ringing experi- 

 ments are such as to indicate that all these substances and others also are 

 translocated mainly by means of this tissue. Czapek 2 has shown that the 

 cortical parenchyma of woody plants is unable to transfer carbohydrates in 

 sufficient amount even for a short distance. 



1 Lecomte, Ann. d. sci. nat., 1889, vii. ser., T. x, p. 300; Strasburger, Ban u. Verrichtung d. 

 Leitungsbahnen, 1891, p. 916. 



2 Czapek, Her. d. Hot. Ges., 1897. p. 124; Sitzungsb. d. Wien. Akad., 1897, Bd. cvi, Abth. i, 

 P- '55- 



