323 



was unable to ascertain. If so, this aerenchyma could not, of 

 course, be considered as '* floating " tissue. 



Its occurrence in this position, as well as on the submerged 

 bases of the upright stems and on the upper part of the larger 

 roots, seems to speak against the assumption that it serves as a 

 floating tissue, for it seems singular and a great waste that it 

 should be produced under circumstances which preclude its acting 

 as a float But if we consider that tlie tissue zones of newly 

 formed lateral organs are continuous with those of the parent 

 stem, we have good reason to assume that the phellogen-Iike 

 meristem of the floating stem extends into the branches and roots. 

 Nor is it improbable that, like other meristems, it may retain its 

 activity for a considerable time, and give rise to additional aeren- 

 chyma, although its original services are no longer required. 



However, I will not venture to assert that, even in NescEa, 

 the spongy tissue does not contribute at all to the aeration of the 

 submerged parts, as, according to Schenck, really seems to be the^ 

 case with many other plants ; but I might say, in conckision, that 

 in reference to NescBa its principal and most valuable function 

 seems to be the assistance it renders the plant to keep its stems 

 afloat, thereby materially facilitating its vegetative propagation. 



November, 18S9. 



EXPLANATION OF THE PLATES. 



Fig. I.— ^y^j.^^ veriicillaia in the beginning of August ; bi^ bottom of pond ; sf^ 



surface of water. 



**IG. 2. — Portion of cross-section 8 cm. from apex of floating stem ; c, cambium ; 



st, sieve-tissue ; ///, phellogen-like meristem ; bf, bast-fibres ; ch, air- 

 channels, X280. 



riG, 3, — Portion of cross-section of young floating stem, fl, floating tissue ; other 



letters as above, x 125. 



IG. 4. Portion of longitudinal section of stem, somewhat nearer apex than Fig, 



3. Letters as above, x 125, 



*'^- 5- — Portion of longitudinal radial section of floating stem, showing the four 



youngest tiers of the floating tissue, x 125, 



l^s. 6 and 7. — Transverse and radial longitudinal sections of rhizoma, cf. text, 



p. 318; en, endodermis, x 125, 

 Fig. 8,— Portion of floating tissue in old stem ; xg4. 



