OFTH ^ ; V\ 



TY ) 

 CHLOROPLASTS J 47 



watery part about the hilum contracts more than the less watery 

 superficial layers, and a crack is the result. 



Now add water so as to dilute the acid : the grains will slowly 

 swell, as with potash, the cracks disappearing first, as the water 

 gains access to the grain. 



Optical Properties. Mount some starch-grains (potato) in 

 water ; examine under a microscope fitted with polarizing 

 apparatus, as described above (p. 41). It will be seen that when 

 the field is dark the grain is bright, and presents a well-marked 

 dark cross ; when the field is bright, the dark cross is replaced 

 by a bright cross. The starch-grain is thus shown to have the 

 property of double refraction. 



The relation of the interference colours can be more definitely 

 made out in starch-grains than in cell-walls. In order to study 

 this point, mount some starch-grains (potato) in water ; rotate the 

 analyzer so that the field is red. Assuming that the starch-grain 

 under examination is so placed that its long axis is directed away 

 from the observer, it will be seen that there is a red cross on the 

 grain corresponding in position to the dark cross mentioned above, 

 that the two lateral segments of the grain are coloured yellow, 

 and that the anterior and posterior segments are coloured blue. 



IV. Chloroplasts, or chlorophyll-corpuscles. 



Mount a fresh Fern prothallus in water, and note as 

 in the last chapter the several parts of the cells which 

 compose it, and especially the green chlorophyll- 

 corpuscles, which are usually of discoid form, sharply 

 defined from the surrounding colourless protoplasm. 



Observe here and there granules of oval or biscuit- 

 shaped outline : these are stages in the process of 

 division, by which means the chlorophyll-corpuscles 

 increase in number. Drawings of a series of such 

 forms should be made so as to illustrate the process of 

 division. 



Treat with alcohol; the green colouring substance 



