34 



resisting positive and negative radial pressure, e. g., diaphragms, 

 "internal hairs," as in Nymphcea, Limnanthenmm, etc., are entirely- 

 wanting in Brasenia. As mechanical, or stereom elements are to 

 be considered only the outer walls of the epidermis cells, the walls 

 of the cells which bound the intercellular canals and at the cir- 

 cumference of the mestom, all of which are somewhat thickened 

 in the manner of collenchyma. 



Returning to the description of the leaf, I have to mention 

 that the spongy tissue underneath the palisade cells fully deserves 

 its name, as the spaces between the *' arms " or rays of the stellate 

 cells are very large. The cell walls enclosing these spaces are 

 thickly covered with a granular, crystalline layer of calcium oxal- 

 ate. Even the elongated cells of the conductive tissue of the 

 fibro-vascular bundles show this calcium oxalate coating wher- 

 ever they border upon the air spaces; the inner walls of the 

 lower epidermis cells, however, are free from it. The conductive 

 system of the leaf is well developed. On a cross-section through 

 one of the strong radiating veins, we see that the bundle is sur- 

 rounded with a starch sheath. The hadrom contains two or 



■ 



three annular or spiral vessels, while the leptom occupies the 

 bulk of the mestom. Very numerous smaller veins, similar in 

 structure, branch off from the principal ones, anastomosing with 

 one another, and forming the typical wavy curves at the margin. 

 Over the middle of many of these curves I found, on the lower 

 epidermis, groups of very small water-pores, the number in each 

 group varying from lo or 15 to as many as 50. (I have also lately 

 noticed water-pores on the lower side of the leaves of our two 

 common species of N^ipkar; In Limnanthemnm I had seen 

 them long since,) The occurrence of water-pores in these plants 

 seems to furnish additional evidence that, even in aquatic plants, 

 the conduction of water is effected chiefly, if not exclusively, 

 through the vascular ducts. The water-pores in Braseftia, being 

 in direct communication with the finest ramifications of the 

 tracheal system, do most likely perform the same office as the 

 water-pores in terrestrial plants, namely : the rapid removal of an 

 excess of water from the conductive tissue — the lower epidermis 

 is made up of flat cells of irregular, deeply sinuate outline, and is 



covered with numerous hairs. 



