AKRANGEMENT OF AIR-CANALS IN STALKS OF NYMPH^AS. 843 



4. Petioles with numerous irregularly disposed canals : tuberosa 

 (sometimes), alba (as in A 3). 

 B. Peduncular air-canals, four, in collateral pairs. 



1. Petioles with four central canals, two large, two smaller (as in 



B 1) : odorata rosea, o. exquisita, o. sulphurea, Laydekeri 

 fulgens, sanguinea, pygmcea helvola, Carolina perfecta, c, nivea. 



2. Petioles with six central canals, two median large, four smaller 



(as in B 2) : Ellisiana, lucida, Marliacea chromatella, Aurora 

 (A 2, B 2). 



3. Petiole with two central canals much larger than the rest (as 



in B 3) : Laydekeri rosea, Marliacea. 



In using this table, full allowance must be made for occasional varia- 

 tions and for the fact that, by rare exception, a particular form might as 

 well be placed in more than one group or in some other group than that in 

 which it is here included. This is specially noticeable in N. alba and 

 N. tuberosa. The hybrid origin of many of the forms is also a possible 

 source of confusion. But after making full allowance for variation and 

 difference of interpretation it will still be found that the arrangement of 

 the air-canals furnishes useful means of discrimination. 



The purport of these canals in conferring lightness with economy of 

 structural material is obvious. Not so clear are the functions of the hairs 

 which so often project from the walls of these tubes. Some are simple, 

 others variously branched, and form beautiful objects under a low power 

 of the microscope. 



