BREATHING PORES. 63 



It is found by suitable tests that the outer and inner faces of 

 the guard cells are thickened and cutinized, while the radial 

 walls are not cutinized and very thin. 



In some cases epidermis is smooth, but in the majority of 

 cases it is roughened by hairs or glands and the walls are wavy 

 in outline. The number and distribution of stomata varies 

 greatly in different epidermal tissues. They are found prin- 

 cipally on the under surface of leaves. In some leaves many 

 are found on the upper surface, in others none at all are found. 



Hold a leaf of the common trailing garden plant, known popu- 

 larly as the Wandering Jew, against the side of a large cork, 

 and with a sharp razor cut off tangentially little patches of the 

 epidermis, both from the upper and lower surfaces. Some of 

 the underlying green cells may be taken with the epidermis, 

 but the edges of the sections will generally consist of epidermis 

 alone. A few of the green cells, in fact, are desirable, as they 

 wilj afford an opportunity to study incidentally chlorophyll 

 granules. - 



Mount a section from the upper surface in water and examine 

 with low and high powers. The epidermal cells are hexagonal 

 in shape, transparent, free from chlorophyll granules and have 

 no intercellular spaces between them. With iodine solution 

 the protoplasm stains yellow and is more readily seen. In all 

 essentials the cells are like those of the onion epidermis. Note 

 that stomata are absent. 



In the same manner examine a section from the lower sur- 

 face of the leaf. The epidermal cells are pretty much the same 

 in appearance, but scattered among them are numerous sto- 

 mata, easily distinguished by the pairs of crescentic cells with 

 dense contents and chlorophyll granules. These cells guard an 

 opening or pore which is readily seen. With iodine solution 

 the granules in the guard cells are very deeply stained. 



Note the relative sizes of epidermal cells and guard cells and 

 whether the latter all point in one direction or not. 



In some of the sections in which the underlying green paren- 

 chyma was cut with the epidermis there may be seen long 

 strands of spirally-marked tubular cells. These are the spiral 

 wood vessels of a small vein of the leaf. This kind of vascular 

 tissue will be studied in a later exercise. 



Examine the epidermis from both sides of a leaf of the Cul- 

 tivated Lily and compare with that of the Wandering Jew. 

 Note the absence of plant hairs from both leaves. 



Mount fragments of a Senna leaf in a few drops of chloral 

 hydrate solution, one with the upper surface turned up, one 



