AQUATIC PLANTS. 
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seed-leaves, in the embryo serves to divide the Angiosperms into the two 
classes, Dicotyledones and Monocotyledoncs . 
It is true indeed — as is well seen in the division last mentioned- that 
the character which gives a name to a group is associated with various 
other difcrenticB by what is termed correlation. Thus we find tliat 
Dicotyledons not only have generally the floral leaves in whorls of five, 
as against the threes among Monocotyledons, but have also generally a 
tap-root and net-veined leaves, as against the tufted fibrous roots and 
parallel venation of the other class. These characters, however, are 
perhaps more subject to exceptions than those from which the classes are 
named ; and when we proceed to the subdivision of these classes we 
again find that botanists have had to adopt characters derived from the 
floral organs, such as the presence or absence of cohesion between the 
petals. These, then, are the characters derived from the more remote 
common ancestors of flowering plants ; whilst we find, on the other hand, 
that the vegetative structures, those concerned directly with the nutrition 
of the individual, such as the forms of leaf and stem, are only distinctive 
of smaller groups, i.e. have been more recently acquired, and are more 
liable to modification by the plant's surroundings. When we take a 
comprehensive survey of those physiological groups of the cultivator to 
which I have alluded, such as alpine plants, succulent plants, or aquatic 
plants, it is precisely in these vegetative organs that we find they most 
agree. We thus arrive at the principle of classification that "the less 
any part of the plant is concerned with special habits the more important 
is it for classification " ; whilst all the characters of a species may be 
divided into those that are inherited — mostly ordinal or generic — and 
those that are adaptations to its environment. 
The chief interest which aquatic plants possess for the scientific 
botanist arises from the fact that closely similar conditions of life acting- 
on plants of the most varied ancestry have produced such remarkable 
resemblances in the general habit, or form and even structure of the 
vegetative organs, as to enable us readily to separate the adaptational 
from the more remotely inherited characters. Though belonging to 
many orders widely divergent in their affinities, as indicated by tlieir 
reproductive organs, aquatic plants form, in fact, a biological group, and 
the study of their adaptation to their mode of life will serve (i.) to show 
that merely physiological characters are no guide to affinity, and (ii.) to 
illustrate {a) the action of the environment on the organism, {b) the 
inheritance of acquired characters, and fc) the varied adaptations of 
structures differing in origin to serve the same functions. 
If we consider the nature of water as an environment or conditioning 
influence upon plant life, we shall the better appreciate the manifold 
adaptations of a most varied assortment of plants to life in such a 
medium. Its high specific and latent heats render water so much slower 
than air to change its temperature that aquatic plants are little liable to 
have their growth checked by such changes, or to be prevented from 
spreading over wide areas within the same zone of temperature. 
The total number of water plants is — at least among flowering plants 
— not large, and their geographical distribution, especially in an east and 
west direction, is far wider than is that of average land plants. Thus, 
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