ii8 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



Algae. Klebs 1 has shown that the presence of i per cent, solutions of inulin, 

 amygdalin, or aesculin enables Conferva to form zoospores in darkness, whereas 

 solutions of grape- or cane-sugar are ineffective. Similarly Hydrodictyon ceases to 

 form zoospores in 0-2 per cent, of Knop's nutrient solution, but rapidly produces 

 them when returned to water 2 . Vaucheria and Chlamidomonas behave similarly, 

 and, as in the case of fungi, algae usually require a higher oxygen-pressure for 

 the formation of sexual than of asexual reproductive organs. Thus Vaucheria is 

 able to grow under an air-pressure reduced to 3 mm. of mercury, but forms 

 normal sexual organs only when the pressure reaches 18 mm. 



Phanerogams. Various changes of shape, and even of structure, are produced 

 as the result of partial or complete starvation, as well as by the action of particular 

 chemical substances 3 . 



PART VII 



THE INFLUENCE OF THE PERCENTAGE OF WATER AND OF TURGIDITY 



SECTION 33. The Action on Vegetative Growth. 



Growth is very closely dependent upon the supply of water, for even 

 a slight fall of turgor causes it to be distinctly slowed owing to the 

 diminished stretching growth of the cell-walls, while it often ceases before 

 the tension of the cell-wall due to turgor is entirely removed. Hence 

 plants drooping for want of water grow but slowly or not at all. 



Externally applied solutions of neutral salts act similarly in retarding 

 or arresting the growth in surface extent of the cell-wall by antagonizing 

 the internal osmotic pressure 4 . Growth may be completely arrested by 

 solutions isosmotic with the cell-sap without any plasmolysis occurring, 

 although the cell is still rich in water, and continues to respire and 

 exhibit active metabolism. Moreover, plasmolysed protoplasts may even 

 be able to form a new cell-wall around themselves, without any subsequent 

 increase in surface extent being possible. 



1 Klebs, Bedingungen d. Fortpflanzung, 1896, p. 351. Cf. also Senn, Bot. Ztg., 1899, p. 97. 

 Starved fern-prothalli also form no sexual organs, Prantl, Bot. Ztg., iSSi, p. 754. 



2 According to Livingstone (Bot. Gazette, 1900, Vol. XXX, p. 289), the mere change in the 

 osmotic concentration of the medium acts in this manner upon Stigeoclonium. 



3 Cf. Frank, Krankheiten der Pflanzen, 1895, 2. Aufl., Bd. I, p. 278. Also Dassonville, Rev. 

 ge'n. d. Bot., 1896, T. vm, p. 284; Bonnier, Compt. rend., 1897, T. cxxv, p. 794; Pethybridge, 

 Beitr. z. Kenntniss d. Einwirkung d. anorgan. Salze, Gb'ttingen, 1899 ; Teodoresco, Rev. g^n. d. Bot., 

 1899, T. xi, p. 445 ; Gottschery, Ann. d. sci. nat., 1899, 8 e sen, T. ix, p. 61. 



4 Cf. de Vries, Median. Ursache d. Zellstreckung, 1877, p. 57; Klebs, Unters. a. d. Bot. Inst. 

 zn Tubingen, 1886, Bd. n, p. 489; Wieler, Ber. d. Bot. Ges., 1887, p. 375; Stange, Bot. Ztg., 1892, 

 p. 353; True, Annals of Botany, 1895, Vol. ix, p. 365; Reinhardt, Bot. Festschrift f. Schwendener, 

 1899, p. 431. 



