EFFECT OF A CCUMULA TION OF ASSIMILATOR Y PRODUCTS 325 



It does not, however, necessarily follow that the auto-assimilatory 

 products can be entirely replaced by an external supply of organic food, for 

 conversely many saprophytic or parasitic plants, which normally obtain all 

 their organic food from without, can be artificially nourished only with 

 difficulty or not at all. Here, as in all cases, it is not merely the quality 

 of the nutriment that is important, but also the form in which it is presented 

 to the plant, the possibility of its absorbtion by the latter, and many other 

 features as well. 



Many plants dependent under normal conditions upon auto-assimilation 

 have been artificially nourished to a certain stage of development (cf. Sects. 

 64, 109), and green leaves may even form starch from an external supply 

 of sugar. This power of depositing starch, when the percentage of sugar 

 reaches a certain limit, is possessed by etiolated chloroplastids, as well as 

 by many non-chlorophyllous chromatophores (Sects. 53, 54). All chroma- 

 tophores have not this power, and certain chloroplastids never contain 

 starch, perhaps because solvent enzymes are present which dissolve the 

 starch as fast as it is deposited (cf. Sects. 54 and 93). 



Starch seems in general to be formed from dextrose and laevulose, 

 and a supply of cane-sugar or glycerine does not always cause it to 

 appear. Such substances as mannite and galactose cause a deposition of 

 starch only in those plants in which these sugars are produced by photo- 

 synthetic assimilation l , while all other substances, including various organic 

 acids, have hitherto given negative results. The formation of starch in such 

 cases is dependent partly upon the rate at which the proffered substance is 

 absorbed and partly upon many other relationships, so that further research 

 in this direction may considerably modify the views at present held. 



Historical. Bohm first showed that chloroplastids formed starch when 

 artificially supplied with sugar, and these results have been considerably extended 

 by A. Meyer, Schimper, Klebs, Laurent, &c. 2 



Methods. Leaves of Phaseolus, Nicotiana, and Mosses, or Algae, such as 

 Spirogyra, which have been kept in darkness or in a medium deprived of carbon 

 dioxide until all starch has disappeared, may be placed in a 10 to 20 per. cent, 

 solution of dextrose, when starch appears in the chloroplastids of algae and mosses in 

 a single day ; in other plants this may not take place till after several days, but in all 

 it may ultimately accumulate to a considerable extent. Allium cepa, however, does not 



Bd. Ill, p. 5; Jost, Jahrb. f. wiss. Bot., 1897, Bd. XXVII, p. 477; MacDougal, Journ. Linn. Soc., 

 1886, Vol. xxxi, p. 526. 



1 A. Meyer (I.e., p. 130) states that starch is formed from mannite in the Olcaceae, from 

 galactose in Sileneae, from dulcite in Eitonymus curopaeus. 



2 Bohm, Bot. Zeitung, 1883, p. 35; Bot. Centralbl., 1889, Bd. xxxvn, p. 200; A. Meyer, Bot. 

 Zeitung, 1886, p. Si ; Schimper, ibid.. 1885, p. 737 ; Klebs, Unters. a. d. Bot. Inst. z. Tubingen, 1886, 

 Bd. II, p. 538, and Bot. Zeitung, 1891, p. 512; Laurent, Bull. d. 1. Soc. Bot. d. Belgique, 1888, T. xxvi, 

 p. 243; Bokorny, Ber. d. Bot. Ges., 1888, p. 116; Versuchsst., 1889, Bd.xxxvi, p. 240; Saposchnikoff, 

 Ber. d. Bot. Ges., 1889, p. 259 ; Acton, Bot. Centralbl., 1890, Bd. XLIV, p. 224 ; Nadson, ibid., 1890, 

 Bd. XLII, p. 48; Bokorny, Biol. Centralbl., 1897, Bd. xvn, p. i. 



