ELODEA STEM 171 



having a denser sclerenchymatous sheath and smaller 

 vascular elements than the former. 



Note the passage of these central bundles outwards 

 to the bases of the leaves : they are common bundles. 

 Note also the mode of formation of the cauline bundles. 



iv. STEM AQUATIC TYPE. 



I. The American Water Weed (Elodea canadensis) may 

 be taken as an aquatic type : it grows habitually sub- 

 merged, with its leaves arranged in whorls, and with 

 elongated internodes. 



Cut transverse sections of an internode : mount some 

 in water or weak glycerine, others in chlor-zinc-iodine : 

 note in the circular transverse sections 



1. The regular epidermis, with no stomata. 



2. The broad band of cortex, with intercellular spaces 

 of various size : six small strands of prismatic cells may 

 be recognized. 



3. The bundle-sheath, consisting of relatively small 

 cells, having the usual dark dot on the radial walls : 

 this will be best seen after treatment with potash. 



4. The central vascular strand, consisting of thin, 

 walled tissue with no lignified walls whatever : cen- 

 trally is a round cavity (vessel), but without lignification 

 of the walls. 



Chlorophyll-granules and starch may be found 

 throughout the tissues. This is evidently a more 

 thoroughly aquatic type than Hippuris, and its com- 

 pletely submerged habit is to be remembered in con- 

 nection with the simpler internal structure. 



