584 TRANSLOCA TION 



a large proportion of the phosphorus and sulphur, and perhaps of potassium 

 also, is stored in the form of organic compounds, but in tubers, roots, &c., 

 the former accumulate mainly as sulphates and phosphates (cf. Sects. 

 74, 77, 80). When mobilized, a portion of the organic compounds which 

 contain sulphur and phosphorus decomposes, liberating phosphoric and 

 sulphuric acids, and these are translocated, along with organic compounds 

 of phosphorus and sulphur and other ash constituents, to the regions where 

 metabolism is active. Both the organic and inorganic compounds appa- 

 rently pass through the phloem and its associated tissues, for during 

 translocation to and from the storage receptacles large quantities of organic 

 and inorganic compounds of sulphur and phosphorus, along with a smaller 

 amount of those of magnesium and still less of those of calcium, are found 

 in the sieve-tubes 1 . A portion of the phosphorus and sulphur is probably 

 translocated in the form of proteids, although both may appear in the cortex 

 without any accompanying accumulation of the latter. Just as in the case of 

 sugar, it is uncertain whether inorganic salts can be translocated sufficiently 

 actively over wide distances through the cortex alone, or whether in all 

 cases the aid of the phloem is necessary. The fact that many parenchyma 

 cells are able to passively secrete sulphates, phosphates, nitrates, &c., would 

 tend to render direct translocation through such tissues more difficult, for 

 before any transference could occur through the tissue all the absorbing 

 cells would need to be completely saturated. 



Every element may be found undergoing translocation in the plant, 

 even including such as calcium or silicon which are for the most part 

 deposited in the form of aplastic compounds, but the readiness with which 

 different substances are mobilized and translocated varies very much. 

 Thus Schroder 2 found that the shrivelling cotyledons of PJiaseolus seed- 

 lings only lost one-half of their calcium, whereas not more than a quarter 

 of the phosphorus, and one-third of the potassium, magnesium, nitrogen, 

 and sodium remained. A complete removal of all the essential elements 

 is never possible, for even in a starved plant certain essential structural 

 constituents cannot be mobilized or consumed. After the death of a cell, 

 however, additional materials may be extracted from it, but these usually 

 consist only of the more readily diosmosing substances, to which compounds 

 of calcium do not for the most part belong. An extraction of this kind 

 always occurs when a dead cell is surrounded by living ones 3 , whereas 



1 Schimper, Flora, 1890, pp. 241, 260. 



- Schroder, Versuchsst., 1868, Bd. x, p. 468. Other examples by Homberger, Jahresb. d. Agr.- 

 Chem., 1882, p. 159; Schimper, 1. c., p. 241, and certain of the works on seedlings, tubers, &c., 

 quoted in Sect. 109. The same occurs during the opening of the bud (Schroder, Forstchem. u. 

 pfl.-phys. Unters., 1878, I, p. 77). 



3 Examples in the case of the xylem : Hartig, Holz d. Rothbuche, 1888, p. 177 j Daube, Bot. 

 Jahresb., 1883, p. 44. 



