BIG 



pelled to follow the rooting internodes down into the water. 

 The formation of the spongy layer surrounding the immersed 

 parts of the plant continues, until in many stems its thickness 

 amounts to 15 mm. and sometimes even more. The greatest 

 thickness is always found at the place at which the swelling 

 started, i.e., in older plants nearest the bottom. Upwards its 

 thickness diminishes until it disappears entirely at the surface of 

 the water. The fissures in the outer bark have, by this time 

 become more numerous, and it adheres to the underlying white 

 tissue only loosely in long shreds, or is thrown off altogether. 



The white spongy mass Itself can be easily separated from 

 the woody part of the stem. On a cross-section numerous con- 

 centric layers can be distinctly noticed even with the naked eye; 

 however they are not separable from one another tangentially, 

 while It is quite easy to tease apart the tissue in thin strips both 

 in a transversely and longitudinally radial direction. On older 

 stems, or when dry. It assumes a light yellowish brown color, 

 retaining its spongy appearance and elasticity. 



Under the microscope we find that this tissue consists of 

 parenchymatic cells of peculiar shape and arrangement. Each of 

 the concentric layers we distinguished with the naked eye or 

 with the aid of a magnifying glass, represents a floor built of 

 cells of equal height but irregular outline, with numerous inter- 

 cellular spaces between them (Figs. 5 and 7). From many of 

 these cells, placed at irregular intervals, arise slender columns of 

 equal height supporting a ceiling which is of the same structure 

 as the floor and which serves as a floor for another tier of col- 

 umns carrying another celling, etc. The further these tiers or 

 stories are removed from the woody part of the stem the larger 

 are the intercellular spaces In the floors, until their cells, having 

 all become quite elongated, form a sort of lattice- work, laterally 

 bracing up the vertical rows of columns (Fig. 7). The shafts 

 of the columns are simply outgrowths of the basal cells on which 

 they stand, so that a longitudinal radial section of such a cell has 

 the appearance of an inverted T with a very long stem. The 

 average length of the columns is 320 // and their width 32 /i. 



The walls of these cells are very thin and consist of cellulose; 

 they contain a delicate lining of protoplasm, in which slow, but 



