STARCHES 113 



association with plastids, in which, as Schimper demonstrated, 

 they have their origin. Not only are the microscopic char- 

 acters of starch grains of diagnostic value, but the different 

 varieties of starch can be grouped into generic, specific, and 

 varietal classes which correspond with the classification of 

 plants based on the ordinary morphological features.* 



Many monocotyledonous plants are characterized by the 

 absence of starch, for example Scilla nutans, Phleum pratense, 

 Allium, etc., but in some of these cases starch granules may 

 occur in the guard-cells of the stomates, in the bundle sheath 

 of the leaves, and also in the bulbs at the base of the growing 

 shoots; further, in certain plants which normally form sugar, 

 e.g. Musa, Hemerocallis, and Muscari, starch will appear 

 when much sugar accumulates. On the other hand, many 

 members of this same class of plants are fairly constant starch 

 producers, e.g. Lilium tigrinwn, Pontederia cor data, Ananas 

 sativa, Canna indica, Tradescantia virginica, Juncus communis, 

 and Alisma Plantago. There are many peculiarities in this 

 occurrence of starch in the Monocotyledons ; for instance, in 

 Scilla nutans it is absent, whilst in Scilla siberica it is quite 

 abundant ; further, the former plant, if fed with cane sugar 

 in a solution of suitable strength, does not form it, while, on 

 the other hand, starch-free plants of Scilla siberica under 

 the same treatment do form starch, the experiment being 

 carried out in the absence of light. In the Mycetozoa, in 

 which starch is normally absent, starch formation may be in- 

 duced under the influence of acid and a supply of sugars, f 

 In the plant starch occurs, as is well known, in the form of 

 variously shaped microscopic bodies composed of concentric 

 layers ; the granules have an organized structure and possess 

 the power of double refraction. 



Preparation of Starch. 



The method of preparation varies according to the source 

 employed. From wheat flour it may be obtained by stirring 

 up this material thoroughly with water, and allowing the mix- 



* Reichert : " Amer. Journ. Bot," 1916, 3, 91. 

 fBoas: " Biochem. Zeitsch.," 1917, 78, 308. 



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