THE MECHANISM OF SECRETION AND EXCRETION 131 



a power of active secretion, may come into play in certain cases 1 . The 

 deposition of calcareous scales on the leaves of many Saxifrages and other 

 plants 2 shows how transpiration may lead to an excretion and external 

 deposition of insoluble substances. Transpiration may also aid in the 

 deposition of silica, which is, however, ultimately dependent upon other 

 factors, and which takes place equally well in diatoms living submerged 

 in water, although no transpiration is here possible. 



All secretions, whether produced by entire members or particular glands 

 (nectaries, hairs, &c.), are products of the vital activity of the plant. The 

 excreted substances may have been absorbed by the plants as such, or may 

 be products of its own secretory activity. In both cases, the conditions 

 necessary for diosmosis and exosmosis may be generated by the plant itself, 

 or the transference of the excreta may take place in other ways as, for 

 example, when a substance which cannot penetrate the protoplast passes 

 from the root to the leaves, where it is excreted entirely in the imbibition 

 water saturating the cell-walls. The study of the excreta still affords 

 unexhausted means of attacking these problems. Thus, special factors 

 must render excretion possible in nectaries from which an abundance of 

 sugar escapes, or in fungal cells which evolve considerable quantities of 

 acids, for these substances may be present within the cell in a more dilute 

 solution than is excreted. 



All protoplasmic secretions which appear externally and are lost 

 to the plant, or which can take no further part in metabolism, are to 

 be regarded as excretory substances. Various secretory products, however, 

 when excreted by the protoplast, may remain in more or less permanent 

 connexion with the organism and attain considerable functional value in 

 its economy. The extra- cellular changes which take place in such sub- 

 stances are also influenced by the relationship existing between mass and 

 chemical reaction : thus the tannate of methyl-blue is decomposed outside 

 as well as inside the cell, provided the least trace of a diosmosing com- 

 pound is formed by the acid with which it is brought into contact (see 

 Sect. 22). Similarly potassium nitrate, when absorbed, is gradually com- 

 pletely converted into a salt of an organic acid, the traces of nitric acid 

 set free being immediately absorbed by the protoplast. To enable 

 a portion only of a compound to be absorbed, the action of an inter- 

 mediary agency is not always necessary, provided that the component parts 

 of the compound are only loosely associated together. Thus when calcium 



1 Volkens, Flora d. aegypt. Wiiste, 1887, p. 27; Ber. d. Bot. Ges., 1887, p. 434; Marloth, 

 ibid., p. 321. Older literature : de Candolle, Physiol., trans, into German by Roper, 1833, p. 206; 

 Meyer, Pflanzenphysiol., 1838, ii, p. 530. 



2 Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIII, p. 514 ; Volkens, Ber. d. Bot. Ges. 1884, 

 p. 334 ; Kohl, Kalksalze u. Kieselsaure in d. Pflanze, 1889, p. 99 ; Anatomical details in de Bary, 

 Comp. Anat., p. 123. 



K 2 



