TRANSLOCATION OF ORGANIC FOOD-SUBSTANCES 581 



form of organic compounds. The sieve-tubes become partially empty during winter, 

 but fill again in spring as the reserve-materials are mobilized, and this is sufficient 

 to show that they serve as translocatory channels and not as storage reservoirs 

 as Blass erroneously supposed'. It is moreover still doubtful whether Fischer's 

 suggestion that the sieve-tubes are especially active in the synthesis of proteids 

 is correct. 



It has yet to be determined whether the cambiform and companion cells 

 function mainly as conducting channels, or simply as intermediaries between the 

 sieve-tubes and surrounding tissues and as temporary storage reservoirs, while the 

 same doubt attaches to the function of the phloem-parenchyma and associated 

 tissues. The presence or absence of fine inter-protoplasmic communications has 

 no importance for the question at issue, for the value of these in translocation is 

 still doubtful (Sect. 20). On the other hand, both non-diosmosing and undissolved 

 substances may pass through the coarse pores in the sieve-plate, although the starch 

 grains often present in sieve-tubes are usually larger than any of the pores (cf. 

 Strasburger, 1. c., p. 478). As the result of this continuity fluid escapes from 

 the sieve-tube when it is cut open, and this usually causes the remaining 

 contents to accumulate in each segment on one side of the sieve-plate. Such 

 currents in mass probably often aid in the transport of materials through the living 

 plant, but it is uncertain whether they are passively produced by accidental pressure, 

 swaying movements, &c., or are due to the activity of the living non-nucleated 

 contents of the sieve-tubes, which are maintained alive and active by means of the 

 nuclear influences radiating from the contiguous cells 2 . In many cases the sieve- 

 tubes become closed in winter by means of a callus covering each sieve-plate, and 

 this callus is dissolved and disappears in spring ; in other plants, however, the sieve- 

 tubes remain open so long as they are living 3 . In most Gymnosperms no coarse 

 sieve-pores are present, while certain mosses and algae possess more or less rudi- 

 mentary sieve-tubes (Haberlandt, 1. c., pp. 308, 334). 



Latex is of comparatively limited distribution, and it is still doubtful to what 

 extent the laticiferous or lactiferous 4 tubes (latex- vessels, latex-cells or intercellular 

 spaces) function under normal conditions as channels for the transport of plastic 

 substances. The latex of many plants contains perceptible amounts of proteids, 

 sugars (5 to 10 per cent, in Morns according to Faivre), fats, and in certain cases 

 starch also, but these are in all cases accompanied by aplastic products, and in 

 other plants the latter are almost entirely pre-eminent. Thus as far as is known 

 (Sect. 87~9o\ india-rubber, resin, ethereal oils, alkaloids, <\rc. are all incapable of 

 further metabolism, and this is also true in most cases of the tannins which 



1 Blass, Jahrb. i. vviss. But., JSyi, Bd. xxn, ]>. ->yo. Cf. also Haberlandt, Physiol. Anat., 

 2. Aufl., p. 288. 



2 Cf. Pfeffer, Sitzungsb. d. Sachs. Ges. d. \Viss., 1896, p. 509. 



3 Anatomical details by Haberlandt,!. c., p. 291 ; Strasburger. 1. c.. p. 927 ; de Bary, Comparative 

 Anatomy, 1882, pp. 191, 210,455. On fungi, de Bary, Morph. u. Biol. d. Pilze, 1884, 2. Aufl. ; 

 Istvanm, Jahrb. f. wiss. Bot., 1896, Bd. xxix, p. 405. 



1 [Nehemiah Grew (Anatomy of Plants, 1682, p. 86) was the first to clearly distinguish between 

 lactiferous vessels and sap-vessels ^wood-tracheae).] 



