I70 METABOLISM 



greater difficulties in the way of the diffusion of substances soluble in water. 

 As a matter of fact, one sees that the aniline dyes mentioned previously pass 

 very rapidly through the outer walls of a cell of an alga, and the cell-walls lying 

 between the parenchyma cells may be compared in their characters with 

 such an algal cell. In the walls of all cells pits occur with great regularity, 

 that is to say, places where the wall remains thin on both sides, and these may 

 serve to shorten the path which the diffusing particles have to take through 

 the membrane. This fact led to the belief that the cell-wall was more difficult to 

 penetrate than the protoplasm. Since we know that the pits are pierced by 

 numerous fine pores, and that by their means not only is the protoplasm of one 

 cell in continuity with that of its neighbours, but that in this way the proto- 

 plasts of the whole plant form one connected system, we must view the pits, as 

 agents for permitting translocation of materials, from an entirely different 

 point of view. It might be imagined in the first place that formed particles 

 of plasma or entire starch grains might be squeezed through these minute canals ; 

 as a matter of fact, Miehe (1901) and Kornicke (1901) have seen even nuclei 

 pass through the membrane, doubtless by way of these protoplasmic bridges, 

 but such migrations, owing to the minuteness of the pores, can be possible only 

 under high unilateral pressures, such as scarcely ever occur in nature. Pfeffer 

 (1892), in investigations specially devised for the purpose, was unable to observe 

 any passage of protoplasm through the pores in the pit-closing membrane. 

 Although the significance of the protoplasmic bridges as agents in the transport 

 of materials in mass is, to say the least, doubtful, still they are obviously of great 

 service in diffusion movements. We may assume that the protoplasm of each 

 bridge consists of the outer plasmatic layer and the inner plasma, so that 

 although the external layer is impermeable, as it is in the cell, still the materials 

 may be able to diffuse through the inner plasma. It is true the canals 

 are very narrow, but they are, on the other hand, very numerous and very 

 short, and from Brown's work (1900) (compare p. 121) we know that, with 

 a suitable arrangement and size of the pores, diffusion can be quite as great 

 as though the entire pit-closing membrane were absorbed. 



According to these determinations it appears that a transference of mate- 

 rials will succeed more easily in long cells in which few partition walls have to 

 be passed through than in short ones. This leads us to consider somewhat 

 more closely the tissues which subserve translocation of materials in the plant. 

 Each normal parenchyma cell can fulfil this function and, as a matter of fact, 

 we find in certain regions that these cells are the only ones carrying out this 

 function. In endosperm parenchyma cells alone are present and other elements 

 are absent from all growing points. But it must be noted that growing pomts 

 exhibit only very slow growth changes and hence a rapid transference of materials 

 is not required. Behind these, where active growth is taking place, tissue 

 differentiation is more manifest, and there we find cells, which obviously are 

 adapted specially to the transport of material, more accurately, of mobile organic 

 substances. These are the sieve-tubes, which, not only from their great length, 

 but also from the partial absorption of their transverse walls by sieve-pores, 

 are peculiarly suited to this purpose. They form long strands lying close to 

 the vascular cords and constitute along with these the ' vascular bundle '. Let 

 us consider, as an illustration, the emptying of a leaf that has been assimilating 

 all day, and inquire as to the part played by the sieve-tubes in the process. 

 SCHIMPER (1885) made an interesting investigation on Plantago, in which type 

 it is possible to remove the vascular bundle from the leaf stalk without causing 

 excessive injury, the leaf meanwhile remaining attached to the stem. Schimper 

 found that a leaf so treated could transfer its starch into the stem in the dark, 

 and he imagined he had discovered in the elongated cells which surround the 

 vascular bundle the so-called ' bundle-sheath ', the conducting organs for sugar 

 translocation. On the other hand, Czapek (1897) pointed out that, although 



