588 TRANSLOCATION 



By these means different substances may be separated from one 

 another, and the translocating materials restricted to a given path, for 

 the tendency to irregular diffusion after diosmosis through the protoplast 

 is counteracted by the absorbent activity of the cells capable of passive 

 secretion. The seedling draws reserve-materials from the seed in the same 

 manner, and similarly in a starved plant the young growing organs may 

 withdraw the food-materials they require from the less retentive older 

 leaves, and so induce premature leaf-fall l . A similar antagonism exists 

 between the phloem and the neighbouring tissues, and hence it is only 

 when the former is completely saturated that the latter receive and passively 

 secrete the superfluous translocatory materials, although even then an 

 accumulation is possible only where a power of passive secretion exists. 

 It is for the latter reason that no starch or glucose can be detected in 

 the epithelial cells of the scutellum of Triticum vulgarc, although they 

 are active agents in the transference of food-materials from the endosperm 

 to the seedling 2 . This forms what is known as an interrupted translocatory 

 channel 3 . In the primary meristem the non-accumulation of starch or sugar 

 is frequently due merely to the absence of any power of passive secretion, 

 but in other cases is owing to the rapidity of consumption. 



Living protoplasts have apparently the power of modifying their 

 permeability from time to time in various ways, and it is possible that 

 not only may solid particles and oil-drops be ingested, but that dissolved 

 non-diosmosing substances may be passed through the protoplast in a 

 similar manner. The power of diosmosis is moreover not solely dependent 

 upon the size of the molecules of the diosmosing substance, for many 

 colloids are readily absorbed and excreted (cf. Chap. iv). Within the 

 tissues a direct transference from cell to cell is possible by means of the 

 fine interprotoplasmic connexions, but it is improbable that these are 

 utilized to any marked extent in the transport of nutrient materials, 

 for the passage from cell to cell is probably sufficiently rapid for all 

 requirements if the cell-walls are normally permeable. Indeed, the large 

 quantities of food-material conveyed to the seedling from the endosperm 

 pass from cell to cell in this manner, and the transference still remains 

 active even if direct contact between the scutellum and endosperm of 

 a maize seed is interrupted by the interposition of a piece of filter paper 

 or a thin film of gypsum. The coarser plasmatic connexions of the 



1 Cf. Sects. 93, 106. The withdrawal of ash constituents from the older organs was proved by 

 C. Sprengel, Die Lehre vom Danger, 1839, P- 47- A similar transference occurs during the repeated 

 tuber-formation of potatoes grown in darkness. Cf. Schacht, Ber. iiber d. Kartoffelpflanze, 1 856, p. 6 ; 

 Hanstein, Sitzungsb. d. Niederrh. Ges., Feb. 3, 1871, &c. On transference of water, cf. Sect. 34. 



2 Sachs, Jahrb. f. wiss. Bot., 1863, Bd. Ill, p. 248. 



3 Several such observations are mentioned by Sachs, 1. c. Cf. also de Vries, Landvv. Jahrb., 

 1879, Bd. vin, p. 444. 



