130 PRACTICAL BOTANY 



been exposed to the light, under conditions suitable for 

 assimilation : but they may be seen with special ease in 

 Fern prothalli which have been thus exposed to bright 

 sunlight for some hours, and then bleached in alcohol. 



Mount such a bleached prothallus in water, or in 

 weak glycerine : examine under a high power, and note 

 the bleached chlorophyll -corpuscles, or chloroplasts, 

 in which highly refractive granules may often be 

 seen. 



a. Stain with iodine solution : the chloroplasts will 

 assume a dusky bluish colour, the blue tint being more 

 or less distinctly localized in the highly refractive 

 granules (starch-grains) above noted. 



b. The presence of the included starch-grains may 

 be more clearly demonstrated by causing them to swell : 

 this may be effected in various ways. 



i. Mount in glycerine and iodine, and warm : the 

 high temperature will swell the starch, which 

 will at the same time stain with the iodine, 

 ii. Treat with potash, and after carefully washing 



out the alkali stain with iodine, 

 iii. The best method is, however, that introduced by 

 Schiniper. Treat the bleached specimens for 

 some hours with a solution of iodine in chloral 

 hydrate : the included starch-grains are simul- 

 taneously swollen and stained blue. 

 III. Since the structure of the leaf of the Sunflower 

 is not a universal type, it will be well to study also the 

 structure of some other leaves ; for instance, the ever- 

 green leaves of the Holly (Hex Aquifoliwn) : note first 

 the short, almost cylindrical petiole, and the leathery 

 lamina with cartilaginous margin and spines. 



