248 STARCH IN THE LEGUMINOS^ 



on its physical constitution, as it is a structure of sphero- 

 crystals. From the optical point of view, starch behaves 

 as if it were formed of small crAstals arranged in close 

 radial lines and shining round a centre which is tiie 

 nucleus or hilum. Such is the explanation given by 

 M. Dubard of the phenomenon of the black cross observed 

 in numerous varieties of starch. Each grain is split up into 

 two dark helds, which intersect at the hilum and which 

 increase in size from the centre to the periphery; the re- 

 mainder of the grain appears, on the contrary, in full light. 

 This polarization enables the position of the hilum to be 

 determined, although it is not always visible. It is to be 

 noted that some starch grains do not polarize, and even in a 

 given variety some polarize and others do not. This is 

 probably a question of the degree of maturitv of the grains. 



Amylase and acids both convert starch into glucose. 



According to most writers, the morphological characters 

 may be classed under the follo\\ing headings : — 



(a) Size of the grains, 



(b) Shape, 



(c) Visibility, shape, arrangement of the nucleus and of 

 the striations. 



In our experiments on starches it- lias not been possible 

 for us to give the dimensions of the grains, the characters 

 of these latter being so dissimilar. 



Owing to the size of the grains, sometimes varying within 

 very wide limits, the average dimension is a very arbitrary 

 figure and would give absolutely no information as to the 

 exact dimensions of the grains. Neither does the measure- 

 ment of the extremes, i.e., of the largest and smallest grains, 

 ofifer any advantage, as one or other of the two may pre- 

 dominate. We have, therefore, been content to sketch them 

 WMth the aid of the camera lucida, retaining the same eye- 

 piece and objective for all. In this way w^e get a standard 

 of comparison which enables us to appreciate the different 

 proportions of the grains in the LeguminoScE examined. 



