312 



THE FOOD OF PLANTS 



solution, or 0-2 to 0-5 per cent, solution of neutral meat extract. For prolonged 

 experiments water or dilute sugar-solution is preferable, for in meat extract the 

 bacterial products soon exercise an injurious effect upon the cell or tissue examined '. 

 The forms known as Spirillum (S. undula and tenue) may also 1 be used, but these 

 are so exceedingly sensitive that unless the utmost care is taken there is considerable 

 danger of experimental error. Spirillum is attracted only by a feeble supply of 

 oxygen, and is repelled wherever a high partial pressure of this gas exists. 



As a test for carbon dioxide assimilation Beyerinck has employed the phos- 

 phorescence of certain bacteria, which is dependent upon a supply of free oxygen. 

 The glowing of phosphorus, the conversion of haemoglobin into oxyhaemoglobin, 

 and the oxidation of reduced indigo carmine to indigo blue, have been used for 

 the same purpose 2 . In every case all free oxygen must be removed at the outset 

 from the object employed for the experiment. 



SECTION 53. The Structure and Properties of the Chloroplastid. 



The chlorophyll bodies belong to the group of plasmatic organs or 

 plastids (Sect. 7 and 8), known as chromatophores 3 , which are derived 



FIG. 45. Spirogyra sjxc. (x 180). 



FIG. 44. (A) Cell from leaf of Vallis- 

 neria spiralis (XA.SO); (ff) and (C> 

 are hignly magnified chloroplastids 

 from the leaf of Selaginella marUnsii. 

 (B) in surface view, (C) in optical 

 median section. The oval bodies in 

 the interior are starch grains. 



FIG. 46. Zygnema cruciatutM(xlk>o\ 



only from their like. Hence they increase only by the division of pre- 

 existent plastids, and may subsequently differentiate into chloroplastids, 

 chromoplastids, or leucoplastids 4 , as the case may be, while the colourless 



1 Cf. Pfeffer, Unters. a. d. Bot. Inst. z. Tubingen, 1888, Bd. n, p. 589 ; Ewart, Bot. Centralbl., 

 1897, Bd. LXXII, No. 9. 



8 Beyerinck, Bot. Zeitnng, 1890, p. 744; Boussingault, Ann. d. sci. nat, 1869, v. se"r., T. x, 

 P. 330 (Phosphorus) ; Hoppe-Seyler, Zeitschr. f. physiol. Chem., 1879, Bd. n, p. 425 ; Engelmann, 

 Pfluger's Archiv f. Physiol., 1888, Bd. XLII, p. 186 (reduced haemoglobin); Beyerinck, Bot. 

 Zeitung, 1890, p. 742 (reduced indigo). 



* [This term is here applied to all plastids which have the power of becoming pigmented if 

 certain conditions are fulfilled. Hence chromatophores may at times be colourless.] 



* For details see Zimmermann, Pflanzenzelle, 1887, p. 45, and the collected literature given by 



