317 



distinct currents may be noticed, and also small rounded starch- 

 grams. The numerous intercellular spaces are filled with air, 

 which makes the tissue appear white. It clings very tenaciously 

 to the outside of the cell-walls, which form a complicated system 

 of capillary tubes, but is easily driven out when thin sections are 

 immersed in alcohol for a short time. 



The manner, time and place of origin of this tissue, and the 

 presence of a great amount of air in it, seem to leave no doubt 

 that its function is to keep the apex of the stem from sinking 

 below the surface of the water, and to keep the stem afloat so 

 that it can continue its growth in the air and sunshine, while at 

 the same time, by sending roots through the water down into the 

 sou, it can establish itself as an independent individual for the 

 next season. In the fall, after the seeds have become ripe, the 

 long, slender stems die, except those portions that have produced 

 the floating tissue around them and have rooted in the mud. 



The dead stems being very brittle, easily break off and soon dis- 



appear. 



The new root-stock thus estabUshed, sometimes at consider- 

 able distance from the mother plant, has developed a woody zone 

 several times as thick as that of the slender stem of which it was 

 a part. All the wood-cells, both of parenchymatic and prosen- 

 chymatic form, are densely packed with reserve starch; the grains 

 are of a very characteristic elongated ellipsoidal form, from 8 to 

 1 6 }x long and i to 2 // wide. Calcium oxalate is found in the 

 pith and bark In almost all its forms; there are raphides, rhom- 

 bic prisms, and globular conglomerations of crystals. The 

 presence of a tannin Is demonstrated by the black stains invari- 

 ably appearing on the razor used to make sections. 



The development of the floating tissue is as interesting as its 

 lunction. When we examine the top of a stem soon after it has 

 touched the water, we find that, about one or two millimeters 

 irom the apex, a large-celled central pith Is surrounded by a cir- 

 cle of fibro-vascular bundles each containing a few annular and 

 spiral ducts and, on the peripheral side, some tender cribrose 

 tubes with horizontal sieve-plates ; no bast elements can be de- 

 tected as yet. The primary bark consists of rounded cells which, 



even at this early stage, have rather large intercellular spaces 



