SUNFLOWER LEAF 79 



lower side ; it has the same characters as the upper layer, but 

 stomata are more frequent. Note the two small guard-cells 

 of each stoina, and below them the large air-chamber. 



Hairs as before seen on the upper surface. Note the 

 mucilaginous walls of these hairs. 



Included starch-grains. These maybe observed in the cells 

 of the mesophyll of any leaf which has 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 re- 

 fractive 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 dis- 

 tinctly 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, to 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 evergreen leaves of the 

 Holly (Ilex aquifoliuni) : note first the short, almost cylindrical 

 petiole, and the leathery lamina with cartilaginous margin and 

 spines. 



Cut transverse sections of the petiole of the Holly : mount 



