382 DK. A. B. RENDLE — SYSTEMATIC 



leaf a similar epidermis surrounds a very large-celled mesophyll consisting only of a 

 single layer at the edges, becoming two-layered as we advance inward, and separated 

 towards the middle line on each side by a large intercellular space. A slender central axis 

 similar to, but smaller than, that of the stem runs up the middle line, and is surrounded 

 by one or two layers of mesophyll-cells. 



The remaining species, forming the subgenus Caulinia, are even simpler in structure. 

 An epidermis closely resembling the underlying cortical layer surrounds a cortex con- 

 sisting generally of one or two outer layers, which is connected with an inner layer 

 surrounding the stele by slender bands separating large intercellular spaces (see PI. XLI. 

 fig. 137, and PI. XLII. fig. 149) ; the connecting-bands are, for the most part, only one cell 

 thick. In Najas horrida (PL XLII. fig. 187) we find a more substantial cortex approaching 

 that typical of N. marina. The leaves, which show some variation in structure, consist 

 generally of two layers separated at a greater or less distance from the midrib by a larger 

 or smaller intercellular space. The midrib comprises a narrow small-celled axis, like that 

 of the stem, and surrounded by two layers of cells. Single cells, or groups of a few, in 

 various positions may become thickened and fibrous supporting-cells ; such developments 

 may be constant for a given species, or, on the other hand, variations may occur within 

 its limits. 



The longitudinal intercellular spaces in the leaf are interrupted by transverse septa, 

 to the strong development of which are due the horizontal lines running from the midrib 

 towards the margin, which are characteristic of some species and occur on older leaves 

 in others. 



The flower, as Magnus as shown, originates in the following way : — A protuberance 

 arises at the growing-point in the axil of the lower leaf of each pair before the appearance 

 of the upper leaf. This protuberance becomes divided by a vertical furrow into a slightly 

 larger (and lower) and a slightly smaller rudiment. The former develops into a flower, 

 the latter into the axillary branch, at the base of which the flower is apparently borne in 

 the mature condition. Sporogenous tissue develops in the apex of the rudiment, which 

 becomes in the female the nucellus of an ovule, in the male the anther. In the female 

 an annular wall grows up round the developing ovule ; its margin becomes 2-3-lobed, 

 according to the ultimate number of the stigmas, and, when fully developed, forms a 

 closed chamber containing the ovule. The time and mode of origin of the outer envelope 

 in N. indica and its allies have not been observed. 



The integuments of the ovule arise, first the inner, then the outer, after the ovary-wall 

 has become partly developed. 



In the male a perfectly symmetrical ring arises round the base of the anther, and 

 subsequently a second appears inside the outer and remains always closely adherent to 

 the anther-wall, which, like itself, consists of a few (2) cell-layers. As Magnus states, 

 and as Campbell has recently indicated in his account of the morphology of N. flexilis 

 (' A Morphological Study of Najas and Zannichellia; 1897), there is no sign of a midrib 

 or of the division of these sac-like outgrowths into leaves. I have also examined many 

 specimens, but have not seen any such indication. 



This floral development is of great interest. Pirst, the flower is produced by the 



