510 HEV. GEORGE HENSLOW ON A THEORETICAL 



The Foliage of Plumules compared. — The partial arrest of the 

 primary leaves of the plumule in the NymphceacecB would seem 

 to explain another contrast which Robert Brown first pointed 

 out. In his observations upon Cycadece in his ' Prodromus,' he 

 says: — "Plumulse autem evolutio cum priori classe [i. e. Mono- 

 cotyledons] magis convenit ; in his omnibus enim, Grramineis 

 Aroideisque exceptis, foliola primaria hujus organi semper 

 abortiva et squamuliforinia, vel merse vaginse sunt, dum in 

 Dieotyledonibus omnibus folia perfecta evadunt " (p. 347). It 

 is not clear why he excepts Grasses, for of this order he 

 remarks : — " Plumula nuda vagina propria inclusa, foliolis pri- 

 mariis in folia perfecta mutatis " (p. 168). This " proper 

 sheath " or " pileole " is simply the first leaf of the plumule 

 itself. Of the Avoided, he notices with regard to the plumule : 

 " Plumula 2-3-phylla, foliolis margine involutis, exteriore coty- 

 ledoni opposito ejusdem rimse marginibus amplexo, plus minus 

 denudato ! interiora opposita amplexante " (p. 334). It would 

 seem, therefore, that in this respect Aroidece would stand alone ; 

 Bentham and Hooker, it may be added, regard this order as a 

 very isolated one. 



The peculiar feature alluded to above of the primary leaf or 

 leaves of the plumule being more or less arrested and scale- or 

 sheath-like in endogens, receives its interpretation from an aquatic 

 habit, as revealed by both Nymphceacece and Sagittaria. 



Rhizome. — The peculiar anatomy of the rhizome of the Nym- 

 phceacece long ago caught the attention of botanists. Trecul, who 

 devoted a year's study to Nuphar luteum * alone, came to the 

 conclusion that " the structure of the stem is what prevails in 

 the greater number of plants which have one cotyledon " ; and 

 from a further study of Victoria regia he expressed himself more 

 positively, in that he was " confirmed in his opinion of the 

 analogy of structure that exists between Nymphceacece and Mono- 

 cotyledons." 



Hooker and Thomson thus write : — " Commencing with our 

 own analysis, we found that the rhizomes presented a central me- 

 dullary mass, surrounded by a tolerably well-defined zone of 



quently the sap, which must have been accumulating and rising as the tem- 

 perature of the spring and early summer increased, now requires extra -sized 

 vessels to carry it off ; the later formed vessels, as, e. g. f of the oak, being quite 

 diminutive in diameter. 



* He describes it by the specific name lutea. 



