xi v] NESAEA 193 



as floating organs. For this particular case, this view can scarcely 

 be maintained, since Goebel 1 has shown that Jussiaea repens 

 floats quite well, even if the roots be all removed. A good case 

 has been made out, however, for regarding the aerenchyma of 

 Nesaea verticillata 2 , one of the Lythraceae, as a true floating 

 tissue. Many of the wand-like stems of the plant, growing on 

 the borders of ponds in America, are described as reaching a 

 length of six to eight feet. In July and August they bend with 

 their own weight until the stem apex touches the water, when it 

 curves upwards again. In the region of contact between the 

 stem and the water a swelling occurs, and roots also arise from 



b.w. 



FIG. 124. Nesaea verticillata, H. B. and K. Plant at beginning of August; s.w., 

 surface of water; b.w., bottom of water;/./., floating tissue. [Adapted from Schrenk, 



J. (1889).] 



this region, anchoring the floating part of the stem to the 

 ground (Fig. 124). The epidermis of the swollen region be- 

 comes fissured, disclosing a snowy white, soft, elastic, spongy 

 tissue, which arises from a pericyclic phellogen. Contraction 

 of the roots draws the swollen part down into the water, and 

 the spongy layer gradually extends over the submerged regions. 

 In the autumn the long slender stems die, except those portions 

 that have produced floating tissue around themselves, and have 

 rooted in the mud. A new root-stock is thus developed, some- 

 times at a considerable distance from the mother-plant. As 

 1 Goebel, K. (1891-1893). 2 Schrenk, J. (1889). 



A. W. P. 13 



