Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 



61 



Sun's energy 



Palisade cell 

 Chloroplost — 



Cuticle 



Upper epidernnis 



?'l I* gases, diffuse toother cells. 



CO2 H2O C6H12O6 



^"^MIM 







spongy tissue 



•Vein 



Air and 

 fluid spaces 



Lower epidermis 



COz enters 



with air 



Guard cells turgid Stonnates 



open in the normal daytime 



condition 



Excess O2 leaves during 

 sugar making 



Excess water (HjO) 

 goes out as vapor 



sroiAUR 



Fig. 4.8. The leaf blade. The essential structures are: the upper and lower cover- 

 ing layers or epidermis; the cells of the palisade and spongy tissue containing the 

 chlorophyll that carries on photosynthesis; the veins that are the highways of trans- 

 portation between leaf and stem (the xylem ducts transport water and the phloem 

 carries food) . Each stoma is a breathing pore leading to the air spaces in the spongy 

 tissue. The guard cells on either side of the pore regulate its size according to the 

 moisture and the amount of oxygen and carbon dioxide exchanged. 



photosynthesis is in full swing. The bean-shaped guard cells are then rotund 

 with stored sugar and water which the sugar has attracted by osmosis. Their 

 plumpness causes them to pull apart and thus to form an opening between 

 them; when they collapse the opening closes. Other conditions within or with- 

 out the leaf affect the guard cells, especially scarcity of water. The stomata 

 are then closed and what water there may be left in the leaf is kept from 

 passing out in transpiration. 



Respiration occurs in all cells of the leaf as it does in the root, the stem and 

 other parts of the plant. Within the green leaf the upper layers of cells hold an 



