THE MECHANISM OF SECRETION AND EXCRETION 133 



conditions it is only particular species that become covered by a deposit of chalk. 

 Since, moreover, all algae assimilate, the removal of the carbonic acid cannot be 

 the sole and primary cause' of the deposition, especially in forms such as Corallina, 

 in which the chalk impregnates the plant throughout, or in Melobesia, which grows in 

 depths where the illumination is relatively feeble. Hassak has indeed shown that 

 in light Chara plants become incrusted with chalk only in water which contains 

 calcium sulphate or chloride 1 . This takes place by means of an alkaline secretion, 

 which however only produces an actual deposition when the carbonic acid is assimi- 

 lated by the plant, for carbonic acid gradually dissolves away the chalky incrustation 

 in water which is not saturated with calcium bicarbonate. Herein lies the necessity 

 for illumination which alone, when calcium bicarbonate is present, must lead to 

 a deposition of chalk, although an incrustation is formed upon the plant only when 

 certain other factors of definite and specific character enter into play 2 . In the 

 presence of calcium salts or of a saturated solution of the bicarbonate, an alkaline 

 excretion will suffice to cause a precipitation of chalk, without any CO. 2 -assimilation 

 being necessary. It is perhaps in this manner that the incrustations on fungi and 

 roots are formed. 



It is in complete accordance with this external origin that the deposition of 

 chalk may be not merely external, but may also take place in the substance of the 

 cell- wall, especially when the outer layers of the latter are gelatinous 3 . Neverthe- 

 less, the possibility remains that both the calcium pectate 4 , and the traces of chalk, 

 which frequently impregnate cell-walls in the interior of the plant, may have a similar 

 metabolic origin. The calcium carbonate found in plasmodia, in cells of the seeds 

 of Celtis, &c., is of internal origin, and the chalk found in the hollow leaf scales 

 of the rhizome of Lathraea may also be a secretory product 5 . The same is the 

 case with the scaly deposit formed on Saxifrages, &c., although here also similar 

 neighbouring plants, in which the drops of water excreted by the water stomata 

 on the leaf-teeth dry up daily, show no such chalky deposit. 



These deposits, incrustations and infiltrations are of importance to the plant 

 in so far as they confer protection and increase rigidity", while from a geological 

 standpoint they are essential for the formation of certain rocks, and indeed may 

 lead to the production of chalk rocks from calcium sulphate 7 . 



1 Hassak, Unters. a. cl. Bot. Inst. z. Tubingen, 1888, Bd. n, p. 465. 



2 Raspail, Nouveau systeme d. chim. organique, 1833, p. 321. Cf. Meyer, Physiol., 1837, Bd. I, 

 p. 126; Hanstein, Sitzungsb. d. Niederrh. Ges., Mai, 1878; Berthold, Jahrb. f. wiss. Bot., 1882, 

 Bd. XIII, p. 710. Pringsheim (ibid., 1888, Bd. I, p. 133) regards the removal of CO 2 as the sole 

 cause, while Bischoff (Die kryp. Gew. Deutschlands, 1828, i. Lief., p. 21), and also Payen (Mem. 

 pies, p. div. savants, 1846, T. IX, p. 78) suppose, on insufficient grounds, that the incrustation is 

 excreted by the plant. 



3 See Leitgeb, Sitzungsb. d. Wien. Akad., 1887, Bd. xcvi, i, p. 21 ; [Church, Ann. of Bot., 

 1895, ix, p. 602]. 



4 Melnikoff, Unters. Uber d. York. d. kohlens. Kalks in Pflanzen, Bonner Diss., 1877, p. 17. 

 See also Kohl, I.e.; Giesenhagen, Flora, 1890, p. 2, and Ber. d. Bot. Ges., 1891, p. 74; Zimmer- 

 mann, ibid., 1891, p. 17. See Mangin, Compt. rend., 1892, T. cxv, p. 260; Rech. anat. s. 1. com- 

 poses pectiques, 1893, p. 48. 



5 Kohl, 1. c. ; Scherffel, Mitth. a. d. Bot. Inst. z. Graz, i8SS, Heft 2, p. 209. 



6 Stahl, Pflanzen u. Schnecken, 1888, p. 70. 



7 Credner, Elemente der Geologic, 1891 7. Aufl., pp. 266, 270. 



