62 VEGETABLE HISTOLOGY. 



usually consists of one, but sometimes of two or three layers of 

 cells. The cells are closely packed together, leaving no inter- 

 cellular space except the breathing pores, and commonly they 

 have that portion of the cell-wall which faces exteriorly consid- 

 erably thickened and cutinized and are usually flattened. When 

 seen in surface view they often appear sinuous or irregular in 

 outline, but sometimes they are straight-sided and regular. 

 In many plants they are somewhat elongated in the direction 

 of the length of the organ, especially in the cells on the veins 

 on the under surface of leaves. 



The cells are rich in protoplasm. The different parts that 

 were noted in the onion epidermis are noticed just as plainly 

 in other cases. In most plants there are no chlorophyll bodies 

 present in the epidermal cells, to which fact is due their trans- 

 parency. Ferns are exceptions to this. 



STOMATA OR BREATHING PORES. 



These pores are minute apertures, usually surrounded by a 

 pair of crescent-shaped cells called guard cells. These are 

 much smaller than the epidermal cells and are much richer in 

 proteid matters, containing a nucleus, protoplasm, numerous 

 chlorophyll bodies and occasionally oil globules. 



By means of the pores the plant exhales the superfluous water 

 taken in by the roots and the excess of oxygen not used by it 

 and takes in the carbon dioxide necessary for the plant's life. 

 They always open into a large intercellular space. Thus the 

 outside air is in free communication with the whole interior 

 of the plant stem and leaves, since the air circulates freely 

 through the intercellular spaces which are in communication. 

 Communication with the interior of the plant takes place only 

 through the pores, since the cutinized exterior of the surface 

 of the epidermal cells is highly impregnable to water and air. 

 Hence epidermis is an excellent protection against evaporation 

 of moisture from the interior of the plant. 



The size of the breathing pore is regulated by the guard cells, 

 which expand or contract according as they absorb or give off 

 moisture. The thin radial walls and the thickened outer and 

 inner walls are so devised by nature that when, in hot, dry 

 weather, moisture is given off by the guard cells to the air, the 

 concave sides enclosing the opening straighten out and thus 

 close it, thereby stopping the further evaporation. Similarly, 

 when the air is moist, the guard cells absorb moisture, and the 

 result is the widening of the pore and any excess of moisture 

 in the interior may escape. 



