VII. — THE MARSH ARROW-GRASS. 
Trig/ochin pa lustre Linne. 
I T is not without significance that the first seven of our selected types of 
Monocotyledons are aquatic or semi-aquatic plants. There is much evidence 
in the structure of the Class that it may be of aquatic, or semi-aquatic, origin. 
The slight tendency to the formation of wood and frequently soft texture, the scanty 
rooting system adapted rather to mud than to a windy situation, the glabrous 
and polished surface, and the entire leaf-margins all point in this direction ; and 
some anatomical evidence has been adduced in support of the suggestion that the 
whole Class may have originated from or near such a semi-aquatic group of 
Dicotyledonous plants as the Ranunculacece , the Buttercup Family. 
Though most of the groups included in the Order Helobie<e , which nearly 
corresponds to Bentham and Hooker’s Cohort Potamales , are aquatic, it must be 
admitted that in structural and physiological characters they seem a somewhat 
heterogeneous assemblage. Among them are the most completely aquatic of 
flowering plants, such as the fresh-water Naias and the Grass-wrack ( Zostera ), so 
familiar on most sea-coasts, both of which are adapted for pollination under water ; 
the large genus Potamogeton , the Pond-weeds, floating or submerged plants flowering 
above water ; the beautiful and fragrant Cape Pond-weed or Winter Hawthorn 
(. Aponogeton distachyum Thunb.), the curious Lattice-leaf of Madagascar (//. 
fenestrale Hook, fil.) ; besides our British Arrow-grasses, Water-plantains, Arrow- 
head, Flowering Rush, and Frog-bit. These British representatives of the Order 
either float on the surface of fresh water, or rise above it, or grow in wet ground by 
the water’s side ; but the Triuridace^e, a small group of Tropical plants included in 
this Order, are saprophytes with little or no chlorophyll or green colouring-matter 
in their leaves, growing on dead leaves in the forest. 
The flowers of the Helobie<e exhibit primitive and variable structural characters 
which suggest that some at least of the Order may be survivals of early 
undifferentiated types ; but it must always be remembered that a certain 
freedom from competition, the equable temperature of the water, and great 
facilities for dispersal of rooting fragments or of seeds have led to a structural 
degeneration in many aquatic plants. In other words, in interpreting the 
structures of such plants it is always important to be on one’s guard against 
considering vestigial structures as truly rudimentary, mistaking degraded higher 
types for really primitive ones. The perianth in this Order may be absent or may 
be represented by one or by two whorls of leaves and, in the last case, the two whorls 
may be similar ( homochlamydeous ) or dissimilar ( 'heterochlamydeous ) : the parts of the 
flower may be partly arranged in spirals ( hemicyclic ) or may be entirely in whorls 
( cyclic ) ; but in all cases they are polysymmetric. By including the Frog-bit Family 
(Hydrocharitace^jWe are also obliged to add that the flowers may be epigynous, although 
