16 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



sharpens more than any other means our power of observation. 

 The learner should first endeavour to represent the object by 

 freehand drawing. He may already possess sufficient drawing 

 ability for this, but if not can readily acquire by practice the 

 needed facility. The object should not be drawn too small, even 

 if the observer believes he sees it very small. A correct judgment 

 on the size of an object in the field of view of the microscope 

 is only attained after long practice, and it is better at first that 

 the learner should draw the object too large, in order conveniently 

 to include in his drawings all the details. No less important 

 is it to provide the individual parts of the figure with suitable 

 distinctive terms, and that the name of the plant, the purpose of 

 the observation, and the most important conclusions, should be 

 clearly noted by the side of the drawing. 



The starch grains of the potato are only slightly flattened, as 

 can be easily demonstrated if, while examining, you push care- 

 fully with a needle against the edge of the cover-glass, and so set 

 the grains rotating. 



Besides the simple grains (as at A, Fig. 5) will be found also, 

 v after some search, semi -compound grains (as in B), which 

 enclose two, rarely more, organic centres or hilums. Each hilum 

 is surrounded by a number of layers peculiar to itself, and both 

 together by a smaller or larger number of common layers. Not 

 infrequently the two linner systems of layers are separated by a 

 cleft, extending -as far as the outer, common, layers (B). The 

 number of layers peculiar to the single grains, as well as of the 

 common layers, varies according to circumstances. 



The completely compound grains, which are found far more 

 commonly than are the semi-compound, consist of two (C), less fre- 

 quently of three (D), rarely of more than three component grains. 

 In contradistinction to the semi-compound grains, the common 

 layers are here wanting. The layers are most strongly developed 

 in the line connecting the centres of the component grains ; these 

 latter, therefore, turn their posterior ends towards, and their 

 anterior ends away from, one another. The line of separation 

 between two component grains often broadens inwards into a 

 cleft. 



For comparison we now put up a preparation of potato starch 

 which has been preserved in an air-dry state, transferring a trace 

 of the powder into a drop of water. As the object-slides may 



