OF CEYLON AND INDIA. 
441 
Tubifloræ, iu the Orchidaceæ, Liliaceæ, Musaceæ, &c. The 
flower of Stellaria media is usually regarded as radial in 
symmetry, but Burkill,'" by examination of a great number of 
flowers, has shown that there is a distinct tendency for the 
upper stamens to be most often absent ; here perhaps we 
have the phenomenon in its incipient stage. 
Again, in the Podostemaceæ the upper stigma is generally 
smaller than the lower, and this is a very common pheno- 
menon among zygomorphic flowers. 
With regard to the immediate mechanism of the production 
of dorsiventral structure, we as yet know but little. 
Vöchting has shown,! in certain cases of slight floral 
dorsiventrality, that rotation upon the clinostat during 
development will entirely prevent the curvatures of the 
organs concerned from taking place. Other phenomena have 
been noticed elsewhere, which tend to show that gravity has 
at least been an active factor in the production of dorsiven- 
trality. In the peculiar thalli of Dicræa stylosa fucoides and 
Griffithella Hookeriana Willisiana above described, the 
thallus is usually drawn out most upon the lower side, while 
it is symmetrical if it grow in an erect position.! Light too 
may have something to do with the production of dorsiven- 
tral structure, by retarding the growth of the illuminated 
side, or in some more direct way. Further physiological and 
experimental morphological work is required before we can 
draw any definite conclusions in regard to the mechanism of 
the production of these or other morphological phenomena. 
In any case, the various possible agents have all a clear field 
for the full exertion of their action in most of the Podoste- 
maceæ, in consequence of the peculiar mode of life of these 
plants. 
It would seem then most probable that to a large extent 
dorsiventrality in the floral organs is due, not to natural 
* Variation in the number of stamens and carpels. Journ. Linn. Soc., 
XXXL, 1895. 
t Ueber Zyg-omorphie Prints. Jahrb., XVII., 1886. 
f And cf. Goebel on Stereum, Flora, 1902, vol. 9Ü, p. 471. 
