8 CHAP. II. EVOLUTION OF PURE OXYGEN UNDER LIGHT 



outside ; the cells in the interior also contain chlorophyll 

 but in less quantities. The noticeable characteristic is the 

 presence of large intercellular spaces, which become filled 



Fig. i. Transverse Section of the Leaf (upper figure), and of the 

 Stem (lower figure) of Hydrilla verticillata 



The layer of larger cells in the leaf is in nature exposed to the 

 sky. Note intercellular spaces in the section of the stem. 



m, midrib ; s, starch-grains ; c, chloroplasts. 



with excreted oxygen during photosynthesis ; from these 

 the gas escapes in a succession of bubblets at the cut end of 

 the stem. The Hydrilla plant grows in lakes, ponds and in 

 watercourses. An economic use was formerly found for 

 it in bleaching the brown Date Palm sugar : layers of the 



