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, II). 



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 

 (PL I, fig. 2 ; PI. VII, fig. 43, 5 h s). 



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 PL VII, fig. 39 ; ^ 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, 



^ Such, an early enlargement of sheath, is mentioned by Van Tieghem 

 (1898, pp. 250-251), who speaks of it as quite general. 



^ Possibly the very low percentages are results of environmental in- 

 fluence, e. g. position of leaf on shoot, surrounding leaf sheaths, &c. 



