4 8 



THE WATERLILIES. 



scar is suberized in a manner identical with that of the root. In one 

 case (Fig. 20 c} the spiro-reticulate elements of a bundle in a leaf scar 

 were closed by large thyloses from the neighboring parenchyma. Since 

 but 3 to 5, usually 3, layers of cork are formed, the protection is only 

 temporary ; and when the water breaks through into the underlying 

 tissues, a thin corky layer is thrown across the cortex in front of it, 

 sometimes simply by modification of the cell-walls without division of 

 their contents. Soon the water breaks through this, and again a partition 

 is thrown across farther forward. So in the older parts of the stem, 

 the plant keeps up a continually losing conflict with death, resulting 

 in a gradually advancing decay of the tissues. 



In the outer, corky 

 -L cells of the scars, and to 

 a less extent in the inner 

 ones, crystals are found. 

 Some of these are irregular 

 and rounded in outline, but 

 most are beautifully rect- 

 angular, prismatic or tabu- 



Fio. 20. (a) Old leaf cushion on rhizome of N. alba candidissima, lar. The regular Crystals 

 showing base of petiole with air-canals, insertion of stipule, s, and 



root scars, r; (h) transverse section of (a) through the two largest may be transparent, homo- 

 air-canals ; (c) thylosis closing spiral vessel in old leaf scar. j i , , 



geneous and sharp-angled, 



or they may be surrounded by a bunch of needles projecting from the 

 middle line toward either end, or they may present much the appearance 

 of bundles of raphides. Similar crystals and crystal masses are found in 

 the suberized exocortex of the older parts of the stem. They are very 

 resistant to reagents, and consist probably of calcium oxalate. They 

 are often imbedded in masses of mucilage which becomes pinkish in 

 chlor-zinc-iodide. The irregular, crystal-like bodies blacken with ferric 

 chloride solution, and doubtless contain tannin. Throughout this 

 research cork has been tested by its yellow color when treated directly 

 with chlor-zinc-iodide, and by a blue color with the same reagent after 

 boiling in potash solution and washing in hot alcohol (Meyer 1898). 



Before taking up the vascular system of the stem it will be of 

 advantage to describe some special types of stem-structure. The 

 remarkably developed stolons of JV. flava and the lesser ones of N. 

 lotus have the same general structure. Inside of a thin but cuticularized 

 epidermis of small cubical cells, and about eight layers (N. flava] of 

 closely fitting cortical cells (two to three layers in N. rubra var.), the 



