158 PLANT PHYSIOLOGY 



by the thickened and strengthened walls. In the sieve-tubes 

 permanent collapse always occurs sooner or later ( as may 

 be seen at any time in the older parts of the phloem in 

 perennials or late in the season in annuals ) . It may be pro- 

 duced at any time by cutting off the part — ^leaf, branch, or 

 stem — whereupon examination wiU reveal the sieve-tubes 

 and their contents in the abnormal condition figured in even 

 the most recent text-books. Collapse of the sieve-tubes un- 

 doubtedly occurs more or less completely, though only tem- 

 porarily, in the healthy plant whenever the leaf or branch or 

 stem is unduly bent and whenever the removal of food from 

 the sieve-tubes by the adjacent cells for use or for storage is 

 more rapid than the suppty of food from the places of man- 

 ufacture. The pressure in the sieve-tubes will vary, just as 

 pressure varies in any cell, according to the prevailing condi- 

 tions; but because the sieve-tubes are continuous and are 

 in contact with cells which consume or store as well as with 

 cells which manufacture food, the osmotic and other pressures 

 in them are likely to be lower than in the food-manufacturing 

 cells. Because they are composed of long cells, the cavities of 

 which are continuous with one another through the pores in 

 the cross-walls, the sieve-tubes are especially adapted to the 

 translocation of the osmotically less portable nitrogenous 

 foods, especially the proteids. It would appear from the 

 investigations of Czapek* that it is especially in them, not 

 even in the companion and cambiform cells of the phloem, 

 that the diffusible carbohydrates also are transferred. It 

 does not necessarily follow, from the discovery of minute 

 starch-grains in the sieve-tubes, that they pass as such 

 through the sieve-like cross- walls. On the contrary, starch 

 occurring in the sieve-tubes must be regarded as carbohy- 

 drate merely temporarily deposited there after being con- 

 verted from portable to stationary form. These starch- 

 grains in the sieve-tubes, like the great majority if not all 

 of the solid particles in other cells, are too large to pass 

 through pores in the cross-walls, f 



" Czapek, F. Zur Physiologie dea Leptoms der Angiospermen. Ber. d. 

 Deutsch. Bot. Gesellsch., Bd. XV. 1897. 

 f Sachs , J. von. Lectures on the Physiology of Plants. Engl, trans. , p. 325. 



