Ii2 



PLANT STEUCTUEES 



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- 



Fia. 122. Cross-section throufrh i portion ot the lut of Pfn i «ho\Mng tbe hcavy- 

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

 each pide of the center) opening into intercellidar passages, the mesopliyll cells 

 containing ehloroplasts, the npper row arranged in palisade fashion, the other 

 cells loosely arranurd (spongy mesophyll) and leavins large intercellnlar passages, 

 and in the centei- a section of a veinlet (vascular bnndle), the xylem being repre- 

 sented by the central group of heavy-wallcd cells.—CALDWET.L. 



ranged, leaving numerous intercellnlar spaces, forming 

 the S2]0iif/i/ fiNsiii'. These spaces form a system of inter- 

 cellular passageways among the working mesopliyll cells, 

 putting them into communication with the outer air 

 through the stomata. The freedom of this communication 



