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| REVUE BRYOLOGIQUE 143 
M. rostratum will rarely fail to show some of these scale-like 
accessory leaves interspersed here and there among the normal. 
leaves, and for the most part fairly constant in form, size: Bad : : 2 
structure. They mark a distinct advance in the evolution of these 
_organs, for it will be noted that they are no longer simply à 
condition of arrested development of the ordinary leaves, their u 
tissue being of quite à different nature from that of the normal 
leaves at any stage of development. They are, I believe, charac- 
teristic of M. rostratum in all its forms, and it would be quite 
possible to determine the species from them alone, so far as the 
European species of Mnium are concerned. 
In the Indian forms of M. rostratum (M. rhynchophorum Hook., 
ete.) they oceur as regularly and perhaps even more clearly 
marked than in our western plant. But in another group of species, 
those included by Wilson in Orthomnion, of which 0. crispum is 
the type, they are still more highly differentiated. An examina- 
tion of the growing point of a creeping stem of O. crispum shows 
two forms of leaf side by side ; first, the young norma Ileaves, me 
nute, orbicular, not at all pointed, and among these, sheathing and à 
more or less enclosing them, somewhat larger and totally diffe- 
rent ones, being triangular-acuminate, usually with a finely acute 
point, with the areolation already fixed and mature, of pellucid, 
elongate cells elosely resembling those of the suborbicular acces- 
sory leaves already described in #. rostralum. They are nerveless 
or show a faint nerve reaching about half-way. These small, tri- ci 
angular or lanceolate-acuminate accessory leaves will be found 
constantly scattered along the creping stem between the ordinary 
leaves in O. crispum, and not only on these but on the erect fertile 
stems and © shoots also. There can be no doubt, I think, that the 
purpose they serve is principally that of protecting the immature 
and tender leaves at the growing point of the stem. This point is 
extending rapidly along the ground, and the extremely delicate 
_ leaves just forming must obviously stand in need of some pro- 
_ tecting covering. In Y. rostratum and M. affine \his is afforded by 
a certain number of the normal leaves (possibly those which 
spring from the lower side of the stem and are more in contaët 
with the ground), which having served this purpose beeome 
supernumerary and arrested in growth, leaving however quite 
sufficient normal leaves Lo form the bifarious arrangement Cha . 
-_ racteristic of these creeping stems. In O.crispum they have become | 
_ specialized organs, already, even in the bud, differentiated and 
_ almost completely developed ; for after being left behind by the 
_ elongation of the stem they undergo scarcely any further deve- 
