President's Address. 18." 5 



of lateral growth ; this is well shown in Archidium alterni- 

 folium. 



Tlie stem and branches are generally more or less 

 leafy, seldom scaly (as in the Ilepatica)), the leaves vary 

 much in their form, and also somewhat in their structure. 

 They generally consist of but one layer of cells similar 

 in structure, sometimes, though rarely, of two kinds of 

 cells, different in form, as in Sphagnum. The leaves of 

 Mosses are generally inserted at right angles to the stem, 

 very seldom parallel with it. Their outline is generally 

 more symmetrical than the leaves of Hepaticse, besides 

 which they are more frequently furnished with one or two 

 nerves, which are rare in Hepaticoe. On the other hand, 

 the genera Bryuvi and Mnium approach in their areolation 

 to the Hepaticse. The cells of the leaves vary much in 

 size, shape, and amount of chlorophyll, of which great 

 advantage is taken in natural arrangements. In Leuco- 

 bryum, and a few others, the green cells are embedded 

 between two layers of colourless cells, thus causing the 

 peculiar glaucous hue which pertains to these plants. The 

 leaves are variable in the presence or absence of a nerve, 

 which may be more or less wide, forked, or absent ; some- 

 times being produced into a more or less excurrent point, 

 which is frequently devoid of chlorophyll, giving the 

 plant a hoary aspect, as in many Racomitrce and Tortulce. 

 In Hypnum illecehrum the nerve is sometimes excurrent 

 about halfway up the leaf, like the awns of some grasses. 

 The nerves of Pogonata and others are covered by small 

 laminae, and in some Polytricha and Tortulce the upper 

 surface of the leaf is partly covered by cellular filaments. 

 In Potia cavifolia the pagina appears to be double or even 

 triple ; there being, in fact, two or three laminal appen- 

 dages growing from the nerve. The margin of some 

 leaves, as Diphyscncm and Hedwigia, are laciniate ; and in 

 others the margin is variously convolute or revolute. A 

 remarkable modification of the pagina is that called 

 equitant, as in Fissidens, the leaves being in two ranks, 

 and the neutral part being double, and sheathing the next 

 leaf. The stems of many Mosses are clothed with variously 

 branched villi, which have been thought to defend them 

 from great heat, but are most probably rudimentary plants. 



