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PLANT STKUCTURES 



organ, its two surfaces being differently related to light. 

 To this different relation the mesophyll cells respond in 

 their arrangement. Those in contact with the upper epi- 

 dermis become elongated and set endwise close together, 

 forming the palisade tissue; those below are loosely ar- 



Fig. 122. Cross-section through a portion of the leaf of Pteris, showing the heavy- 

 walled epidermis above and below, two stomata in the lower epidermis (one on 

 each side of the center) opening into intercellular passages, the mesophyll cells 

 containing chloroplasts, the upper row arranged in palisade fashion, the other 

 cells loosely arranged (spongy mesophyll) and leaving large intercellular passages, 

 and in the center a section of a veinlet (vascular bundle), the xylem being repre- 

 sented by the central group of heavy-walled cells. — Land. 



ranged, leaving numerous intercellular spaces, forming 

 the spongy tissue. These spaces form a system of inter- 

 cellular passageways among the working mesophyll cells, 

 putting them into communication with the outer air 

 through the stomata. The freedom of this communication 



