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. 

 5jt 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, 



FIG. 18. Stomata (st.) on leaf of the garden beet, capable of SO 

 Moderately magnified. See also Figs. 15, 19 and 



22. 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 



