42 



PRINCIPLES OF PLANT CULTURE 



of the hardier varieties of apple, as the Oldenburg (Duch- 

 ess), have more numerous and more crowded palisade 

 cells than less hardy varieties.. Compare the palisade 

 cells of a leaf of the Oldenburg apple (Fig. 17) with 

 those of Fig. 3, which shows a section from a leaf of a 

 tender variety of apple. 



65. Breathing pores or stomata. — Minute openings 

 commonly occur through the epidermis of leaves and young 



stems of plants. 

 5,t 

 / 



St- -^^--^^~—^- 



5t~-_ 



These connect 

 the intercellular 

 spaces (Fig. 17) 

 of the interior 

 of the leaf with 

 the external air 

 and thus serve 

 as breathing- 

 pores. Each 

 such pore is 

 bounded by a 

 pair of cells, 

 guard-cells, 

 capable of so 

 changing as to 

 open or close 

 the passage. The pore with its guard-cells is called a 

 stoma (plural stomata). Stomata are chiefly found on 

 the lower side of leaves, and are extremely numerous, but 

 are too small to be seen without the microscope. An av- 

 erage apple leaf has been computed to contain about 

 150,000 stomata to the square inch on its lower surface. 

 The guard-cells are attached together only at their ends 

 and are so thickened on the inner side that they become 



Fig. 18. — Stomata (al.) on leaf of the garden beet. 

 Moderately magnified. See also Figs. 15, 19 and 

 22. 



