THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 171 



there was no reason to doubt the conducting power of these cells, still they 

 were not sufficiently extensive to carry out the transport of the whole of the 

 materials ; he held that this was the chief task of the sieve-tubes. Although 

 CzAPEK was unable to advance any definite proof of his view, nevertheless it 

 would seem highly probable that this is the case. He made incisions into 

 the leaf-stalk of Vitis on a summer evening, cutting through half of the vascular 

 bundle, and found in the morning that the part of the lamina which was thus 

 partially isolated was still filled with starch, but that the other portion was 

 empty. Since, in this experiment, not only the sieve-tubes but also the bundle 

 sheath was cut through, one cannot draw any conclusion against Schimper's 

 view ; we can only conclude that the general fundamental parenchyma of the 

 petiole is not sufficient for the translocation of the products of assimilation, 

 and further that this transference cannot be effected in a transverse direction, 

 and hence that elongated elements are necessary. The same conclusion 

 is arrived at from experiments in ringing trees. If from an unbranched 

 stem we remove a ring of cortex, right into the wood, the part of the 

 tree situated below the region of ringing is not filled with starch, whilst 

 the part above the ringing accumulates it abundantly. If a narrow bridge of 

 cortex be retained, connecting the upper with the lower part of the tree, the sugar 

 flows backwards but spreads laterally very slowly. If the bridge be in the form 

 of a step ( h ) no backward passage of the products of assimilation takes place, 

 since the horizontal part of the bridge does not conduct them. The wood region 

 in the tree is unable to carry the products of assimilation backwards, and, 

 in the cortex, conduction takes place only in the longitudinal direction. This 

 may depend on the special capacity of the cortical parenchyma ; it is more 

 likely, however, that it is the sieve-tubes that are the organs of conduction. 

 [Haberlandt (1904) has, however, advanced arguments against this view.] 



If we accept Czapek's hypothesis as to the function of the sieve-tubes we 

 obtain the following picture of the translocation of carbohydrates from the 

 organs of assimilation. The sugar arising from the transformation of starch 

 eventually reaches the sieve-tubes after migrating through the assimilating 

 cells and the bundle sheath. G. Kraus (compare Pfeffer, Phys. I, 592) 

 found 38 per cent, of the dry weight of sieve-tubes consisted of soluble carbo- 

 hydrates. In the sieve-tubes it is capable, by mechanical means, by streaming 

 of all kinds, of travelling rapidly for long distances. The sieve-tubes may, 

 under certain conditions, form a strand several centimetres or even decimetres 

 in length, which operates just like a single cell ; by diffusion it receives the 

 sugar in at the upper end and gives it off at the lower. Movement in the 

 intermediate region seems not to be effected by protoplasmic streaming, since 

 such streaming appears to be absent in sieve-tubes (Strasburger, 1891, 363) ; 

 still we may conceive of a movement in mass, caused by varying osmotic 

 pressures in the surrounding parenchyma. An exudation of contents due to 

 pressure of neighbouring cells may indeed be observed when sieve-tubes are cut 

 across. We must not, however, suppose the function of a phloem strand to be 

 merely to serve as a means of communication between two regions some dis- 

 tance apart, as though these were in connexion by means of a glass tube. On 

 the contrary, the sieve-tubes during their entire course are in lateral connexion 

 with the phloem parenchyma and give over to them all surplus carbohydrate, 

 and these, owing to vigorous starch formation, are always ready to absorb new 

 materials. The parenchyma which lies in contiguity with the sieve-tube, acts 

 as a storage tissue, and, indeed, as in the case of trees, in a double sense. In 

 the first place, certain reserves are deposited in them, as in the parenchyma 

 of the medullary rays, of the cortex, and of the wood, for next spring ; but 

 starch, not only in the stem, but in every petiole, is also deposited in the 

 phloem parenchyma as so-called temporary reserve, that is to say, the surplus 

 inflowing sugar wanders out of the sieve-tubes and from time to time may 



