EEVISION OF THE GENUS NAJAS. 381 



The leaves are apparently opposite ; each pair consists, however, of a lower and an upper 

 leaf on opposite sides of the stem. The edges of the sheatli of the lower leaf overlap the 

 sheath of the upper, which is amplexicaul. Examination of the gTowing-point shows that 

 the upper leaf appears later than the lower, and at an appreciahly liigher level. Each pair 

 forms a sharp angle with the one immediately above or below. Branching occurs only in 

 the axil of the lower leaf, from the second pair following the cotyledon onward. In 

 vegetative shoots, as Magnus has shown, the lowest leaf on the branch is reduced to a 

 small scale bearing a vegetative bud in its axil, whereas in fertile shoots a tlower 

 occupies the place of this scale-leaf wdth its bud. The lowest developed leaf of the 

 branch (i.e. the upper of the lowest pair), in w^hich no bud arises, forms with the 

 adjacent pair of leaves on the main axis an apparent whorl of three. 



The leaves are sessile, the leaf-base forming a well-developed sheath, sharply dis- 

 tinguished from the blade. The sheath may appear truncate, in its natural am- 

 plexicaul position, from its widely-rounded shoulders, the sloping edges of which overlap ; 

 it may have more or less sloping shoulders ; or it may be drawn out into longer or 

 shorter auricles. The up]3er margins and auricles are generally more or less broken with 

 small teeth or longer spine-bearing processes, the outgrowth in every case ending in a 

 characteristic sharp yellowish-brown spine. The form of the sheath is generally constant 

 for individual species and often for larger groiips, and affords useful specific characters. 

 Sometimes, however, as in N^ajas indica, the form varies widely even on the same 

 plant (see PI. XXXIX. figs. 34-37). 



Within the sheath is a pair of minute hyaline cellular scales, which are generally 

 subulate or filiform, or flat tapering from the base. Their shape is very variable, even on 

 the same plant. They occur in connection with the cotyledon. 



The leaf-blade may spread at a broad angle from the sheath or be more or less con- 

 tinuous with, and ascending from, it. It is generally narrow -linear, tapering above to 

 the apex, and sometimes becomes setaceous ; rarely is it broader and linear-lanceolate. 

 The margin varies considerably. In the broader-leaved forms of iV^ marina it is sinuate- 

 dentate, the teeth often exceeding in length the leaf -width, and ending in a strong 

 yellow-brown spine with a base of several strengthening cells. There is every gradation 

 between this and the structure in N. graminea^ where simple marginal cells protrude in 

 the form of ascending yellow-brown translucent spines visible only under a lens, the 

 leaf appearing entire to the naked eye. In N. marina and several of its varieties, teeth 

 similar to those on the internodes are found on the midrib on the back of the leaf. 

 Similar dorsal spines occur in W. lacerata. As A. Braun indicated, the size and structure 

 of the marginal teeth afford useful specific characters. 



The internal structm'e of stem and leaf is extremely simple. It is most developed in 

 iV. marina, where a well-defined small-celled epidermis surrounds, in the stem, a many- 

 layered parenchymatous cortex, the middle layers of which are separated by intercellular 

 spaces. A narrow axial stele is bounded by a phloeoterma which shows well-defined 

 endodermoid markings on the radial w^alls. The stele consists of much narrower, closely- 

 packed parenchyma and a central cavity, which just below the growing-point is occupied 

 with thin elongated cells and may be considered to represent tracheal elements. In the 



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