62 
The Podostemaceae of India and Ceylon . 
floriferous; the vascular tissue, leading to them, which up to that 
elements and the surrounding cortical tissue becomes thick-walled, 
time is as usual totally undifferentiated, now develops typical xlyem 
whilst the remaining unaltered portions of the thallus soon wither 
or fall away entirely. By the development of these conducting 
elements, the floral shoots, which are no longer submerged at the 
time of flowering, are supplied with the necessary food-materials and 
moisture. 
One of the most important points, brought out by Mr. Willis’s 
paper, is the gradual transition from radial to dorsiventral floral 
structure, which can be observed in the Podostemaceae, and which 
can scarcely be accounted for on the principle of natural selection. The 
flowers of Tristicha ramosissima show radial symmetry, but in the 
other genera, in which the thallus exhibits such a marked dorsi¬ 
ventral structure, this also becomes more and more apparent in the 
floral organs, just as it does in the secondary shoots. This 
dorsiventrality of the flower evinces itself first in the suppression of 
certain of the floral organs and leads up to the extremely dorsi¬ 
ventral flower of Farmeria metzgerioides, in which one of the two 
loculi of the ovary is much more strongly developed than the other, 
which is practically abortive. If anything this zygomorphism of the 
flower is a positive disadvantage, and cannot be regarded as having 
arisen as an adaptation to the mode of life. Mr. Willis considers it 
to be the result of correlation in relation to the vegetative organs, 
and thinks that the dorsiventrality of the flowers has been forced 
upon them by the dorsiventrality of the former. It is at 
least remarkable, that the more flattened and dorsiventral the vegeta¬ 
tive organs are, the greater is the degree of zygomorphism evident in 
the flower. Lawia forms an exception to this rule, in that, 
although the vegetative organs exhibit an extreme of dorsiventrality, 
the flower itself is radially symmetrical in structure. 
The author also extends this view of the origin of dorsiventrality 
in the flower of the Podostemaceae to the zygomorphic floral 
structure of the other orders and points out that correlation may in 
many cases have done more towards determining the structure of 
an organ than natural selection. 
F. E. Fritsch. 
