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A TEXTBOOK OF THEORETICAL BOTANY 



separated by membranes only one cell thick. These lacunae are limited in 

 length to one internode in Dicotyledonous stems, and even in Mono- 



FiG. 860. — MyriophyUiim spicatiim. Trans- 

 verse section of stem showing central 

 stele and large radial air spaces in the 

 inner cortex. 



I 



Cortex 



Stele 



Fig. 861. — Potomogeton sp. Transverse section of 

 stem showing reduced stele with one central 

 vessel and irregular air spaces in the ground 

 tissue. 



cotyledons they are not of great vertical length, but the horizontal membranes 

 which separate them are perforated by minute intercellular passages, which 

 form at all the angles of the cells. These provide for the movement of air 



