The Morphology of Ruppia Maritima. 95 
is well exemplified in the leaf sheaths of Ruppia. It is stated by 
Goebel (1898, p. 556) that no general rule can be given for the 
time of stipular development, but that as Massart (1894) had already 
emphasized before him, they are formed and called into play when 
their function is most needed, as is in fact true of other parts of 
the leaf (Goebel, 1898, p. 503). I shall undertake presently to show 
why the leaf sheaths may be considered stipular as to their nature 
and origin. 
In Ruppia the delicate parts in the region of the growing point 
are in great need of the protective offices of the early developed 
sheaths of surrounding leaves; and also the mature leaves, since 
they almost always contain axillary buds, require large enveloping 
sheaths. These considerations may explain the manner of sheath 
development. 
d. Morphological and Morphogenetic Considerations. 
The nearly related genus Potamogeton throws considerable light 
upon the question of the morphology and morphogeny of the leaf 
sheath. This genus, as is well known, embraces a great variety of forms, 
from those with broad floating leaves only (or with the addition 
merely of capillary phyllodia), passing through intermediate forms 
which have both narrow submerged leaves and broad floating 
leaves, to the typical submerged plants with narrow leaves only. 
In the last group we find basal sheaths on the leaves of most of its re- 
presentatives, e.g. P. filiformis, P. pectinatus, P. interruptus, &c; but 
in the broad leaved forms and in the intermediates we find no 
sheaths, free stipules, however, being more or less prominent. 
It seems clear, then, that in the submerged forms sheaths replace 
the stipules; and this deduction is borne out by such a species as 
P. diversifolius, which has the submerged leaves with stipules 
sometimes adnate, and even better by P. spirillus, where the stip- 
ules are always adnate to the submerged leaves, while those of the 
floating leaves in both species are free. 
A comparison of any of the above mentioned typical submerged 
Potamogetons with Ruppia—especially P. pectinatus and P. filiformis 
—shows the similarity of the sheaths in the two genera. One may 
conclude, then that both the free stipules of the Potamogetons 
and the sheathing stipules of the submerged forms and of Ruppia 
are Closely related, being connected as they are by many gradations; 
and it is probable that the sheathing stipule has been evolved from 
the free stipule, such as it is in Potamogeton. 
As to the reason why this form of stipules is developed in water 
plants, the most obvious explanation seems to be that joined to 
