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EEVISION OF THE GENUS NAJAS. 385 



same time, the pollen-grains, when set free, fall, owing to tlieir greater specific gravity 

 from their richness in starch, on to the stigmas heneath. Magnus (in Engl. & Prantl, 

 POanzenfam. ii. pt. 1, 216) thinks this unlikely, as the male flowers stand erect in the leaf- 

 axil and the anther breaks through the apex of the spathe and dehisces at the top, so 

 that the pollen would not fall directly upon a female flower heneath. But as he himself 

 has figured for Nojas tenuifolia, and as occurs in other species, the spathe may be split 

 laterally and the anther, as I have seen in dried specimens, becomes pushed out beyond 

 the sheath by the curved pedicel (see for instance PL XLI. fig. 120, N. Kurziana, and 

 fig. 142, N.foveolata). 



The fruit is narrowly ellipsoidal or oblong, enveloped, where this occurs, in the 

 persistent spathe, and bearing the remains of style and stigmas. In fresh specimens of 

 N. marina and N.jjraminea the pericarp-cells were turgid, forming a succulent coat ; in 

 the dried specimens the wall becomes very thin and membranous, generally clinging so 

 closely to the seed as to take the impressions of the pitting on the testa. The seed has a 

 eonspicuous raphe which makes it slightly asymmetrical. The seed-coat is hard and 

 brittle when dry, consisting in 1^. marina of many layers of cells witli hard, thickened, 

 pitted Avails, surrounded on the outside by a row of very large cells with thin walls 

 and clear contents, the side-walls having a delicate reticulate thickening. The raphe is 

 distinguished by a group of thin-walled cells and several layers of large hyaline cells on 

 the exterior. Ultimately the large, thin-walled, outer cells perish, and the testa consists 

 of a stone-parenchyma, the surface of which is rugulose with irregular polygonal pittings. 

 The thickness of the testa varies considerably in the different forms and varieties of the 

 species. There may be as few as 4 layers of stone-cells or as many as 8 to 10. The 

 innermost layers become much compressed tangentially by the growing embryo. 



In the remaining species which form the subgenus Caulinia the testa consists, as 

 Magnus as shown, of three layers, and shows three types, depending on the mode of 

 development of the outermost layer. The cells of the innermost layer are thickened 

 and become much flattened tangentially. Those of the middle layer are more nearly 

 isodiametric and have very thick, hard, much pitted walls. In N. flexiUs and N. tenuissima 

 those of the outer layer become similarly thick-walled and pitted, and the seed has a 

 smooth polished surface. In a second type, which is also the commonest {N. minor, 

 N. graminea, N. foveolata and allies), the outer layer consists of large, clear, thin-walled 

 cells with a delicate spiral thickening on the side-walls. The cells vary in shape in 

 different species, giving the characteristic areolation to the testa. Thus in N. minor 

 they are transversely elongated, giving a ladder-like apj)earance, in iV. graminea isodia- 

 metrical. In a third tyj)e the outer and side-walls of the outermost layer are not 

 thickened, but collapse, while the inner wall becomes thickened, and with its concave 

 surface forms the shallow pits, rows of which give a characteristic marking to the seed- 

 coat. Magnus j)oints out that in N. microdon both the second and third form may occur 

 on the same seed. 



The large straight embryo completely fills the testa. It consists of a large hypocotyl 

 and radicle, a well-developed lateral plumule, and a blunt terminal cotyledon. It was 

 correctly described by Richard in 1811 (in Ann. Mus. Hist. Nat. xvii. 233). 



