NAIADACEAE 507 



2949. P. natans L. (Knuth, op. cit., p. 137.) In this species the closely- 

 crowded spike consists of some 50 flowers about 



4-5 mm. in diameter. In the first (female) stage it 

 is about 4 cm., in the second (male) one about 6 cm. 

 long, and just projects from the water. The four 

 green perianth-like connectives are at first closed like 

 an envelope, with only the four brush-shaped stigmas 

 projecting. These are dusted by means of the wind 

 with pollen from adjacent flowers already in the male 

 stage. The connectives then diverge, and the eight 

 sessile anther-lobes dehisce and empty their abundant f\g. ^17. Potamogreionuafans, 



1 ^ 11 L. (from nature). Flower in the 



QUSiy pOliCn. g^.^^ (fgjuale) stage, seen from 



2950. P. perfoliatus L. (Warnstorf, Verh. bot. tZ\ /' r^SK "T^L 

 Ver., Berlin, xxxvii, 1896.) The stalks of the connec- (x about 6). 



tive-plates of the protogynous wind flowers belonging 



to this species are vertical to the ovary when the pollen is mature, while their ex- 

 panded parts are parallel to it. In this way is constituted an excellent arrangement 

 for catching the pollen carried by the wind. The pollen-grains are ovoid to ellipsoidal, 

 white in colour, slightly transparent, delicately retiform-tuberculate, about 44-50 yx 

 long and 37-5 /a broad. 



2951. P. crispus L. (Warnstorf, op. cit., xxxviii, 1896.) The flowers of this 

 species are protogynous and the anthers extrorse. The pollen-grains are white, 

 spheroidal to ovoid, almost glabrous, from 37-47 yu. in diameter. MacLeod (Bot. 

 Jaarb. Dodonaea, Ghent, v, 1893, pp. 284-5) gives a detailed description of the 

 flower mechanism, corresponding for the most part with my account of P. natans. 



2952. P. lucens L. (Warnstorf, op. cit.) The pollen-grains in this species 

 are whitish in colour, irregularly tetrahedral, from 25-8 //, in diameter. 



2953. P. gramineus L., van (a) gramineus Fr. (Warnstorf, op. cit.)- The 

 pollen-grains in this species are white in colour, irregularly tetrahedral, almost 

 retiform-tuberculate, from 31-5/11 in diameter. 



2954. P. pusillus L. (Warnstorf, op. cit.) The pollen-grains in this species 

 are flour-white in colour, tetrahedral, closely tuberculate and opaque, varying in size, 

 on an average 25 /a in diameter. 



2955. P. filiformis Pers. (Abromeit, ' Bot. Ergeb. von Drygalski's Gronlands- 

 exped.,' p. 78.) Vanhoff'en observed nearly ripe fruits of this species in Greenland. 



966. Ruppia L. 



Flowers hermaphrodite, hydrophilous, and usually protandrous. The pollen- 

 grains possess no cxtine. 



2956. R. maritima L. (= R. spiralis Dumort.). (Delpino u. Ascherson, 

 ' Corrispondenza ' ; H. Schenck, 'Biol. d. Wassergewachse,' p. 123 ; Roze, Bui. soc. 

 bot., Paris, xli, 1894, pp. 466-80.) The flowers of this species possess no perianth, 

 and consist of two stamens and four carpels. The spadix is made up of only two 

 such flowers, situated on opposite sides of the axis. In the first stage the spadix is 

 male; it is then short and scarcely projects from the sheath of the bract. The 



