The Morphology of Ruppia Maritima. 93 
filiformis (Sauvageau, 1891, II, p. 259, and Prillieux, 1864) are very 
similar, the main difference being the absence in both of the mar- 
ginal bundles, although Althenia has in place of these, small groups 
of fibrous cells. Althenia is, moreover, further distinguished by a 
rather large number of lacunae of various sizes. 
The leaves of Zostera, Phyllospadix, Posidonia, Cymodocea and 
Halodule, show greater differences, of all of which, together with 
the remaining Potamogetonaceae, Sauvageau has made an excellent 
comparative study in his “ Feuilles des monocotyledones aquatiques ” 
(1891, Il). 
2. The Stipular Sheaths. 
a. Structure. 
At every node the leaf envelops the stem by means of basal 
sheaths composed of only two layers of cells. These cells lack 
chlorophyll and become very minute at the free edge of the sheath 
feel, ne. 2; Pl. VIL, fis. 43, sks). 
b. Development. 
I have already (p. 89) mentioned the fact that the development 
of the sheath rudiments in the young leaf primordium is quite 
marked. From the record of measurements (see Table p. 94) of 
the length of sheaths at different stages of leaf development there 
is manifested a certain periodicity in the growth of the sheath. 
Thus its percentage of the whole leaf length in a very early stage 
is high, as is shown in Nos. 1 and 2 and PI. VII, fig. 39;1 next, 
during the special growth of the leaf (see p. 89) this percentage 
noticeably decreases, in some cases to a very marked extent ;? finally, 
due to the ultimate basal intercalary growth, the proportional size 
of the sheath again increases until in the mature leaf the proportional 
length of sheath to entire leaf averages about 1 to 6 (Table, Nos. 
27-38). 
Although, even by making considerable allowance for error, the 
data in the table are not at all uniform, as is natural, yet they do 
bring to light with no uncertainty the large comparative develop- 
ment of sheath both at the beginning and at the end of leaf growth. 
c. Function. 
That this periodicity in sheath development is related to the 
principal function of stipules, that of protection of younger parts, 

1 Such an early enlargement of sheath is mentioned by Van Tieghem 
(1898, pp. 250-251), who speaks of it as quite general. 
2 Possibly the very low percentages are results of environmental in- 
fluence, e.g. position of leaf on shoot, surrounding leaf sheaths, &c. 
