The Morphology of Ruppia Maritima. 95 



is well exemplified in the leaf sheaths of Ruppia. It is stated by 

 Goebel i'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 deHcate parts in the region of the gro^\ing point 

 are in great need of the protective offices of the early developed 

 sheaths of surroxinding leaves ; and also the mature leaves, since 

 they almost alwa3s contain axillar}" buds, require large enveloping 

 sheaths. These considerations may explain the manner of sheath 

 development. 



d. Morphological and Morphogenetic Considerations. 



The nearh' related genus Potamogeton throws considerable light 

 upon the question of the morphology' and morphogeny of the leaf 

 sheath. This genus, as is well knowni, embraces a great variety- of forms, 

 from those with broad floating leaves onl}^ /or with the addition 

 mereh" of capillar}' phyUodia , passing through intermediate forms 

 which have both narrow submerged leaves and broad floating 

 leaves, to the t\-pical submerged plants with narrow leaves onlj*. 

 In the last group we find basal sheaths on the leaves of most of its re- 

 presentatives, e. g. P. filiformis, P. pectinatus, P. interruptiis, &c: but 

 in the broad leaved forms and in the intermediates we find no 

 sheaths, free stipiiles, 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 b}' such a species as 

 P. diversifolius, which has the submerged leaves with stipules 

 sometimes adnate, and even better b}' P. spirillus, where the stip- 

 ules are always adnate to the submerged leaves, while those of the 

 floating leaves in both species are fi-ee. 



A comparison of any of the above mentioned tA'pical submerged 

 Potamogetons with Ruppia — especiall}' P. pectinatus and P. filiformis 

 — shows the similarit}' 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 closeh' related, being connected as they are by mam* gradations : 

 and it is probable that the sheathing stipule has been evolved ft-om 

 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 



