374 



STARCHES OF GRAMINACE^. 



GENUS AVENA. 



lucliKlcd in tiliis goiuis aro about 50 species which are native to the temperate regions of the 

 01(1 and New Worlds. The most important species is ^4. saliva Linn., of which there are riuite a 

 niunber of cultivated forms. The agricultural variety or form known as Clydesdale oats was used 

 as a type of the genus. 



STARCH OF AVENA SATIVA VAR. (CLYDESDALE). (Plate 2, figs. 15 and 16. Chart 20.) 



Histological Characteristics. In form the grains are usually simiile and are in part isolated, 

 and in part in aggregates which consist of two or more componcTits, and in clumps. The conspicu- 

 ous forms of the aggregates, which are very i^rominent in oat starch, are oval, ovoid, and pyriform. 

 The component grains vary in number from two to very manj^ The conspicuous forms of the 

 isolated grains are polygonal, spindle-shaped, oval, ovoid, round, and irregular oval with one side 

 either flattened or concave. The large round grains resemble those of Triticum. 



The hilurn is frequently invisible. It is occasionally observed as a clear round spot, centric 

 or slightly eccentric. Either a nearly roimd cavity or a longitudinal cleft may sometimes be found. 

 Spindle-shaped grains arc often doublets having a cavity at the region of each hilum. 



The lamdlce are not visible. 



The isolated grains vary in size from the smaller, which are 2 by 2/x, to the larger polygonal, 

 wdiich are 12 by lOyu, and to the large ovoid and roimd grains, wdiich are about 20 by 18/j. The 

 common size of the isolated grains is 8/i. The aggregates may be as large as 40 by 30/x. 



Polariscopic Properties. The figwc in the smaller grains appears to be centric, but it is so 

 small and polarization so low that the details of its form can not Ixb acciu-ately determined. The 

 figure in the larger grains is centric or slightly eccentric. The lines of the figure aro usually not 

 distinct, excepting towards the margin; and they are rather thick and sometimes straight, but often 

 either bent or bisected. 



The degree of "polarization is low in most of the gi'ains, but rather high in a few. It frecpiently 

 varies in the same aspect of a grain. The different quadrants in the small grains often show a vari- 

 ation, and in the majority of the larger grains the 

 polarization of the margin is higher than of the central 

 portion, and it may be entirely or almost entirely 

 absent. Occasionally it is rather high throughout the 

 grain. It is higher when the grain is viewed on edge. 

 On the whole, it is low. 



AVith selenitc, in the small gi'ains the quadrants are 

 well defined, somewhat irregular in shape, and unequal 

 in size. In the larger grains the quadrants are seldom 

 well defined, and they are irregular in shape and miequal 

 in size. The colors arc not pure; the yellow is propor- 

 tionately brighter than the blue. In a few grains the 

 colors are quite bright and almost pure. 



Iodine Reactions. With a 0.25 per cent Lugol's 

 solution, most of the grains color a very light old-rose 

 to a light reddish-violet; a few do not color at all, while 

 a few color deeply. The color gradually deepens until 

 in 30 minutes most of the grains are cither of a rather 

 dark i-cddish-violet or a fairly deep old-rose. With a 

 0.125 per cent solution the grains color a light reddish- 

 violet, which becomes fairly deep in 5 minutes, and quite deep in 30 minutes. After heating in 

 water until the gi'ains are completely gelatinized, the solution does not color, but the grains assume 

 a deep purple tint on the addition of iodine. After boiling for 2 minutes the solution colors a very 

 deep purple and the grain-residues a light r(>ddish-violet. The capsules assume a deep old-rose or 

 wine color when an excess of iodine is added. 



Staining Reactions. AVith gentian violet the grains begin to stain very lightly at once and in 

 30 minutes they are lightly stained. 



AVith safra7iin the grains liegin to stain very lightly at once and in 30 minutes they are lightly stained. 



Temperature Reaction. The temperature of gelatinization is 62 to 64 C, mean 63. 



Curve of Reaction-Intensities of Starch of Avcna sativa 

 var. (Clydesdale). 



