48 



Irrigation and Drainage 



In the upper part of the figure, the under surface of the leaf 

 is shown covered by its skin or epidermis, through which there 

 can but little evaporation take place except through the opening 

 which is shown at sp and the seven others like it ; and even 



these openings or breathing 

 pores are so made that they 

 may be automatically opened 

 wide or almost completely 

 closed when the needs of the 

 plant call for much or little 

 air. 



In the lower part of the 

 figure, the skin has been re- 

 moved from the leaf, so as to 

 show the actual breathing sur- 

 face of the barley plant, con- 

 sisting of the cells marked m, 

 and which are filled with the 

 green coloring matter of the 

 leaf, or chlorophyll. The open 

 spaces, marked *, between the 

 breathing cells, are the breath- 

 ing or respiratory chambers, 

 which communicate with one 

 another all through the leaf, 

 but under the cover of its 

 skin or epidermis, which in various ways, by a varnish, a wax or 

 a close mat of hairs, is rendered less pervious to water and 

 to air. In the case of tall plants, like shrubs and forest 

 trees, rising a hundred and more feet into the air, nature has 

 made still greater efforts to avert the danger of plants being 

 destroyed by the action of drying winds. Here we find the 

 trunks and all the larger limbs thoroughly protected by a thick 

 bark, through which there can but little water escape as it slowly 

 ascends from the roots to the leaves ; indeed, the more detailed 

 we make the study of the structure and the function of parts in 

 the plant, the more plain it becomes that in most land plants the 



Pig. 5. Structure of barley leaf. (After 

 Sorauer.) sp is a breathing-pore; m, 

 chlorophyll cells ; i, respiratory cham- 

 bers. 



